hotfix: blockchain/backend: Skip genesis leaf to unblock syncing (#5103)

This PR effectively skips over cases where the blockchain reports the
genesis block as leaf.

The issue manifests as the blockchain getting stuck and not importing
blocks after a while.
Although the root-cause of why the blockchain reports the genesis as
leaf is not scoped, this hot-fix is unblocking the new release.

While at it, added some extra debug logs to identify issues more easily
in the future.

### Issue

```
2024-07-22 10:06:08.708 DEBUG tokio-runtime-worker db::blockchain: Checking for displaced leaves after finalization. leaves=[0xd62aea69664b74c55b7e79ab5855b117d213156a5e9ab05ad0737772aaf42c14, 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe] finalized_block_hash=0x8f8e…7f34 finalized_block_number=24148459
2024-07-22 10:06:08.708 DEBUG tokio-runtime-worker db::blockchain: Handle displaced leaf 0xd62aea69664b74c55b7e79ab5855b117d213156a5e9ab05ad0737772aaf42c14 (elapsed 25.74µs) leaf_number=24148577
2024-07-22 10:06:08.709 DEBUG tokio-runtime-worker db::blockchain: Leaf points to the finalized header 0xd62aea69664b74c55b7e79ab5855b117d213156a5e9ab05ad0737772aaf42c14, skipping for now (elapsed 70.72µs)


// This is Kusama genesis
2024-07-22 10:06:08.709 DEBUG tokio-runtime-worker db::blockchain: Handle displaced leaf 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe (elapsed 127.271µs) leaf_number=0
2024-07-22 10:06:08.709 DEBUG tokio-runtime-worker db::blockchain: Skip more blocks until we get all blocks on finalized chain until the height of the parent block current_hash=0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe current_num=0 finalized_num=24148458
```

### Before

```
2024-07-20 00:45:00.234  INFO tokio-runtime-worker substrate: ⚙️  Preparing  0.0 bps, target=#24116589 (50 peers), best: #24116498 (0xb846…8720), finalized #24116493 (0x50b6…2445), ⬇ 2.3MiB/s ⬆ 2.6kiB/s    
   
...

2024-07-20 14:05:18.572  INFO tokio-runtime-worker substrate: ⚙️  Syncing  0.0 bps, target=#24124495 (51 peers), best: #24119976 (0x6970…aeb3), finalized #24119808 (0xd900…abe4), ⬇ 2.2MiB/s ⬆ 3.1kiB/s    
2024-07-20 14:05:23.573  INFO tokio-runtime-worker substrate: ⚙️  Syncing  0.0 bps, target=#24124495 (51 peers), best: #24119976 (0x6970…aeb3), finalized #24119808 (0xd900…abe4), ⬇ 2.2MiB/s ⬆ 5.8kiB/s    
```

### After

```
2024-07-22 10:41:10.897 DEBUG tokio-runtime-worker db::blockchain: Handle displaced leaf 0x4e8cf3ff18e7d13ff7fec28f9fc8ce6eff5492ed8dc046e961b76dec5c0cfddf (elapsed 39.26µs) leaf_number=24150969
2024-07-22 10:41:10.897 DEBUG tokio-runtime-worker db::blockchain: Leaf points to the finalized header 0x4e8cf3ff18e7d13ff7fec28f9fc8ce6eff5492ed8dc046e961b76dec5c0cfddf, skipping for now (elapsed 49.69µs)
2024-07-22 10:41:10.897 DEBUG tokio-runtime-worker db::blockchain: Skip genesis block 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe reporterd as leaf (elapsed 54.57µs)
2024-07-22 10:41:10.897 DEBUG tokio-runtime-worker db::blockchain: Finished with result DisplacedLeavesAfterFinalization { displaced_leaves: [], displaced_blocks: [] } (elapsed 58.78µs) finalized_block_hash=0x02b3…5338 finalized_block_number=24150967
2024-07-22 10:41:12.357  INFO tokio-runtime-worker substrate: 🏆 Imported #24150970 (0x4e8c…fddf → 0x3637…56bb)
2024-07-22 10:41:12.862  INFO tokio-runtime-worker substrate: 💤 Idle (50 peers), best: #24150970 (0x3637…56bb), finalized #24150967 (0x02b3…5338), ⬇ 2.0MiB/s ⬆ 804.7kiB/s
2024-07-22 10:41:14.772 DEBUG tokio-runtime-worker db::blockchain: Checking for displaced leaves after finalization. leaves=[0x363763b16c23fc20a84f38f67014fa7ae6ba9c708fc074890016699e5ca756bb, 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe] finalized_block_hash=0xa1534a105b90e7036a18ac1c646cd2bd6c41c66cc055817f4f51209ab9070e5c finalized_block_number=24150968
2024-07-22 10:41:14.772 DEBUG tokio-runtime-worker db::blockchain: Handle displaced leaf 0x363763b16c23fc20a84f38f67014fa7ae6ba9c708fc074890016699e5ca756bb (elapsed 62.48µs) leaf_number=24150970
2024-07-22 10:41:14.772 DEBUG tokio-runtime-worker db::blockchain: Leaf points to the finalized header 0x363763b16c23fc20a84f38f67014fa7ae6ba9c708fc074890016699e5ca756bb, skipping for now (elapsed 71.76µs)
2024-07-22 10:41:14.772 DEBUG tokio-runtime-worker db::blockchain: Skip genesis block 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe reporterd as leaf (elapsed 75.96µs)
2024-07-22 10:41:14.772 DEBUG tokio-runtime-worker db::blockchain: Finished with result DisplacedLeavesAfterFinalization { displaced_leaves: [], displaced_blocks: [] } (elapsed 80.27µs) finalized_block_hash=0xa153…0e5c finalized_block_number=24150968
2024-07-22 10:41:14.795 DEBUG tokio-runtime-worker db::blockchain: Checking for displaced leaves after finalization. leaves=[0x363763b16c23fc20a84f38f67014fa7ae6ba9c708fc074890016699e5ca756bb, 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe] finalized_block_hash=0xa1534a105b90e7036a18ac1c646cd2bd6c41c66cc055817f4f51209ab9070e5c finalized_block_number=24150968
2024-07-22 10:41:14.795 DEBUG tokio-runtime-worker db::blockchain: Handle displaced leaf 0x363763b16c23fc20a84f38f67014fa7ae6ba9c708fc074890016699e5ca756bb (elapsed 39.67µs) leaf_number=24150970
2024-07-22 10:41:14.795 DEBUG tokio-runtime-worker db::blockchain: Leaf points to the finalized header 0x363763b16c23fc20a84f38f67014fa7ae6ba9c708fc074890016699e5ca756bb, skipping for now (elapsed 50.3µs)
2024-07-22 10:41:14.795 DEBUG tokio-runtime-worker db::blockchain: Skip genesis block 0xb0a8d493285c2df73290dfb7e61f870f17b41801197a149ca93654499ea3dafe reporterd as leaf (elapsed 54.52µs)
2024-07-22 10:41:14.795 DEBUG tokio-runtime-worker db::blockchain: Finished with result DisplacedLeavesAfterFinalization { displaced_leaves: [], displaced_blocks: [] } (elapsed 58.66µs) finalized_block_hash=0xa153…0e5c finalized_block_number=24150968
2024-07-22 10:41:17.863  INFO tokio-runtime-worker substrate: 💤 Idle (50 peers), best: #24150970 (0x3637…56bb), finalized #24150968 (0xa153…0e5c), ⬇ 1.2MiB/s ⬆ 815.0kiB/s
2024-07-22 10:41:18.399  INFO tokio-runtime-worker substrate: 🏆

 Imported #24150971 (0x3637…56bb → 0x4ee3…5f7c)
```

Closes: https://github.com/paritytech/polkadot-sdk/issues/5088

---------

Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>