Revert non-best block (#11716)

* Revert non-best block

This makes `revert` also revert non-best blocks.

* Update client/db/src/lib.rs

* Do not count leaves against the maximum number to revert

* Add some explanation

* Fix bug

* Apply suggestions from code review

Co-authored-by: Davide Galassi <davxy@datawok.net>

Co-authored-by: Davide Galassi <davxy@datawok.net>

substrate/client/api/src/backend.rs

@@ -507,7 +507,8 @@ pub trait Backend<Block: BlockT>: AuxStore + Send + Sync {
 
 	/// Attempts to revert the chain by `n` blocks. If `revert_finalized` is set it will attempt to
 	/// revert past any finalized block, this is unsafe and can potentially leave the node in an
-	/// inconsistent state.
+	/// inconsistent state. All blocks higher than the best block are also reverted and not counting
+	/// towards `n`.
 	///
 	/// Returns the number of blocks that were successfully reverted and the list of finalized
 	/// blocks that has been reverted.

substrate/client/api/src/leaves.rs

@@ -259,6 +259,11 @@ where
 
 		removed
 	}
+
+	/// Returns the highest leaf and all hashes associated to it.
+	pub fn highest_leaf(&self) -> Option<(N, &[H])> {
+		self.storage.iter().next().map(|(k, v)| (k.0.clone(), &v[..]))
+	}
 }
 
 /// Helper for undoing operations.