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.