chainntnfs/btcdnotify: fix dropped block epoch notification bug

This commit a bug introduced in the chain notifier while we were
limiting the usage of mutexes within the package. In a prior commit a
default case was introduced in the select statement in order to avoid
the possibility of the main goroutine blocking when dispatching block
epoch notification.

In order to avoid this potentially disastrous bug, we now instead
launch a new goroutine for each client to ensure that all notifications
are reliably dispatched.

chainntnfs/btcdnotify/btcd.go

@@ -476,14 +476,13 @@ func (b *BtcdNotifier) notifyBlockEpochs(newHeight int32, newSha *chainhash.Hash
 	}
 
 	for _, epochChan := range b.blockEpochClients {
-		// Attempt a non-blocking send. If the buffered channel is
+		go func(ntfnChan chan *chainntnfs.BlockEpoch) {
-		// full, then we no-op and move onto the next client.
 			select {
-		case epochChan <- epoch:
+			case ntfnChan <- epoch:
 			case <-b.quit:
 				return
-		default:
 			}
+		}(epochChan)
 	}
 }