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/tests.rs

@@ -139,13 +139,16 @@ impl Backend for TestBackend {
 	fn load_stagnant_at_up_to(
 		&self,
 		up_to: Timestamp,
+		max_elements: usize,
 	) -> Result<Vec<(Timestamp, Vec<Hash>)>, Error> {
 		Ok(self
 			.inner
 			.lock()
 			.stagnant_at
 			.range(..=up_to)
-			.map(|(t, v)| (*t, v.clone()))
+			.enumerate()
+			.take_while(|(idx, _)| *idx < max_elements)
+			.map(|(_, (t, v))| (*t, v.clone()))
 			.collect())
 	}
 	fn load_first_block_number(&self) -> Result<Option<BlockNumber>, Error> {