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/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.