approval-voting: query finalized block on startup and increase look back (#4705)

* approval-voting: add more logs

* approval-voting: query finalized block on startup and increase look back

polkadot/node/service/src/relay_chain_selection.rs

@@ -53,6 +53,8 @@ use std::sync::Arc;
 /// or disputes.
 ///
 /// This is a safety net that should be removed at some point in the future.
+// Until it's not, make sure to also update `MAX_HEADS_LOOK_BACK` in `approval-voting`
+// when changing its value.
 const MAX_FINALITY_LAG: polkadot_primitives::v1::BlockNumber = 500;
 
 const LOG_TARGET: &str = "parachain::chain-selection";