chainHead: Report unique hashes for pruned blocks (#3667)

This PR ensures that the reported pruned blocks are unique.

While at it, ensure that the best block event is properly generated when
the last best block is a fork that will be pruned in the future.

To achieve this, the chainHead keeps a LRU set of reported pruned blocks
to ensure the following are not reported twice:

```bash
	 finalized -> block 1 -> block 2 -> block 3
	
	                      -> block 2 -> block 4 -> block 5
	
	           -> block 1 -> block 2_f -> block 6 -> block 7 -> block 8
```

When block 7 is finalized the branch [block 2; block 3] is reported as
pruned.
When block 8 is finalized the branch [block 2; block 4; block 5] should
be reported as pruned, however block 2 was already reported as pruned at
the previous step.

This is a side-effect of the pruned blocks being reported at level N -
1. For example, if all pruned forks would be reported with the first
encounter (when block 6 is finalized we know that block 3 and block 5
are stale), we would not need the LRU cache.

cc @paritytech/subxt-team  

Closes https://github.com/paritytech/polkadot-sdk/issues/3658

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>