Limit stagnant checks to a certain amount of entries (#5742)

* Limit number of elements loaded from the stagnant key

This will likely be required if we enable stagnant prunning as currently database has way
too many entries to be prunned in a single iteration

* Fmt run

* Slightly improve logging

* Some more debug nits

* Fmt pass

polkadot/node/core/chain-selection/src/backend.rs

@@ -46,10 +46,11 @@ pub(super) trait Backend {
 	/// Load the stagnant list at the given timestamp.
 	fn load_stagnant_at(&self, timestamp: Timestamp) -> Result<Vec<Hash>, Error>;
 	/// Load all stagnant lists up to and including the given Unix timestamp
-	/// in ascending order.
+	/// in ascending order. Stop fetching stagnant entries upon reaching `max_elements`.
 	fn load_stagnant_at_up_to(
 		&self,
 		up_to: Timestamp,
+		max_elements: usize,
 	) -> Result<Vec<(Timestamp, Vec<Hash>)>, Error>;
 	/// Load the earliest kept block number.
 	fn load_first_block_number(&self) -> Result<Option<BlockNumber>, Error>;