Inside publish (for a Serai transaction from the coordinator), use RetiredDb over latest session

Not only is this more performant, the definition of retired won't be if a newer
session is active. It will be if the session has posted a slash report or the
stake for that session has unlocked.

Initial commit towards implementing SlashReports.

coordinator/src/tests/tributary/mod.rs

@@ -30,6 +30,7 @@ mod sync;
 impl PublishSeraiTransaction for () {
   async fn publish_set_keys(
     &self,
+    _db: &(impl Sync + serai_db::Get),
     _set: ValidatorSet,
     _removed: Vec<SeraiAddress>,
     _key_pair: KeyPair,