Client attempts to reconnect during shutdown

When shutting down the GELF Client, it immediately attempts to reconnect, which results in the following error message:

> INFO org.graylog2.gelfclient.transport.GelfHttpTransport - Channel disconnected!

The same issue happens for the `transport.stop()` and `transport.flushAndStopSynchronously()` methods.

No ill effects occur, but this should be cleaned up. Reconnects should not be attempted after calling either of the stop methods.