netsync: Don't request recently removed mix msgs.

This modifies the logic that determines if a mixing message should be
requested to avoid requesting recently removed mixing messages by making
use of the newer RecentMessage method that queries both the existing
messages in the mixing pool and recently removed messages.

Author: davecgh

internal/netsync/manager.go

@@ -1551,15 +1551,12 @@ func (m *SyncManager) needMixMsg(hash *chainhash.Hash) bool {
 		return false
 	}
 
-	if m.cfg.MixPool.HaveMessage(hash) {
+	// No need for mix messages that are already available in the mixing pool or
+	// were recently removed.
+	if _, ok := m.cfg.MixPool.RecentMessage(hash); ok {
 		return false
 	}
 
-	// TODO: It would be ideal here to not 'need' previously-observed
-	// messages that are known/expected to fail validation, or messages
-	// that have already been removed from mixpool.  An LRU of recently
-	// removed mixpool messages may work well.
-
 	return true
 }