SocketConnector is now a Callable, all tests pass

LOGBACK-848: refactored SocketAppenderBase

invoke setRunner method in ConcurrentServerRunner so that InstrumentedConcurrentServerRunner's overriden impl can be used in tests

Since the appender now has some an asynchronous task, this is a best
practice.  Moreover, some test runners may not be able to terminate
properly if there are non-daemon threads remaining in the JVM.

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().

Previously, exited only on interrupt, but if the executor is shutting
down, there's no reason for us to wait to be interrupted.