Drain notification registeration channel.

This prevents a hang when attempting to set the wallet (and register
for wallet notifications) when the process is interrupted and the rpc
server begins shutting down.

rpcserver.go

@@ -1174,6 +1174,7 @@ func (s *rpcServer) drainNotifications() {
 		case <-s.minedDebits:
 		case <-s.confirmedBalance:
 		case <-s.unconfirmedBalance:
+		case <-s.registerWalletNtfns:
 		}
 	}
 }