Finality notification: Optimize calculation of stale heads (#11200)

* Finality notification: Optimize calculation of stale heads

While looking into some problem on Versi where a collator seemed to be stuck. I found out that it
was not stuck but there was a huge gap between last finalized and best block. This lead to a lot
leaves and it was basically trapped inside some loop of reading block headers from the db to find
the stale heads. While looking into this I found out that `leaves` already supports the feature to
give us the stale heads relative easily. However, the semantics change a little bit. Instead of
returning all stale heads of blocks that are not reachable anymore after finalizing a block, we
currently only return heads with a number lower than the finalized block. This should be no problem,
because these other leaves that are stale will be returned later when a block gets finalized which
number is bigger than the block number of these leaves.

While doing that, I also changed `tree_route` of the `FinalityNotification` to include the
`old_finalized`. Based on the comment I assumed that this was already part of it. However, if
wanted, I can revert this change.

* FMT

* Update client/service/src/client/client.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Do not include the last finalized block

* Rename function

* FMT

* Fix tests

* Update figure

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/primitives/blockchain/src/header_metadata.rs

@@ -201,6 +201,11 @@ impl<Block: BlockT> TreeRoute<Block> {
 	pub fn enacted(&self) -> &[HashAndNumber<Block>] {
 		&self.route[self.pivot + 1..]
 	}
+
+	/// Returns the last block.
+	pub fn last(&self) -> Option<&HashAndNumber<Block>> {
+		self.route.last()
+	}
 }
 
 /// Handles header metadata: hash, number, parent hash, etc.