netsync: don't ask for blocks from peers on the same block height

When we're all caught up and we have a new peer, we'll ask for blocks
from peers that we're on the same height. Since these peers don't have
any blocks to send us, they don't reply and we disconnect from them as
they timeout.

To prevent this, we don't ask for blocks if the chain thinks we're
current and do not have a peer that reports having a higher block.

netsync/manager.go

@@ -309,6 +309,11 @@ func (sm *SyncManager) startSync() {
 		higherPeers = append(higherPeers, peer)
 	}
 
+	if sm.chain.IsCurrent() && len(higherPeers) == 0 {
+		log.Infof("Caught up to block %s(%d)", best.Hash.String(), best.Height)
+		return
+	}
+
 	// Pick randomly from the set of peers greater than our block height,
 	// falling back to a random peer of the same height if none are greater.
 	//