Allow capping the amount of work performed when deleting a child trie (#7671)

* Allow Backend::for_keys_in_child_storage to be aborted by the closure

* Ext::kill_child_storage now takes an upper limit for backend deletion

* Add Storage::storage_kill_limited() runtime interface

* review: Use a new version of kill_storage instead of a new interface

* review: Simplify boolean expression

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

* review: Rename for_keys_in_child_storage

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

substrate/client/light/src/backend.rs

@@ -441,14 +441,14 @@ impl<H: Hasher> StateBackend<H> for GenesisOrUnavailableState<H>
 		}
 	}
 
-	fn for_keys_in_child_storage<A: FnMut(&[u8])>(
+	fn apply_to_child_keys_while<A: FnMut(&[u8]) -> bool>(
 		&self,
 		child_info: &ChildInfo,
 		action: A,
 	) {
 		match *self {
 			GenesisOrUnavailableState::Genesis(ref state) =>
-				state.for_keys_in_child_storage(child_info, action),
+				state.apply_to_child_keys_while(child_info, action),
 			GenesisOrUnavailableState::Unavailable => (),
 		}
 	}