Using closed connection in the protobuf client

If the remote server closed the connection, the protobuf client will constantly print this error message in the log when it should just drop the connection and wait for the remote server to reconnect:

```
[EROR] (github.com/influxdb/influxdb/coordinator.(*ProtobufClient).readResponses:174) Error while reading messsage size: &{%!d(string=use of closed network connection)}
```