exit from SocketReceiver.run when executor rejects a task

Previously, we exited only when interrupted.  But if the executor is
shutting down, there's no reason for the receiver to try to continue
running.

This was a left over from when the receiver had its own executor
service, and ExecutorService.shutdownNow was used to shut it down on
stop().
added link to twitter qos.ch account
reference to follow_us.png
added unit test
Merge pull request #118 from qos-ch/LOGBACK-848

LOGBACK-848: refactored SocketAppenderBase