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
...