improved SocketAppenderBase

SocketAppenderBase now uses SocketConnector for its (re-)connection
logic, and uses an asynchronous task to dispatch events to the remote
receiver.  A configurable queue is utilized to relay logging events from
the append method to the dispatch task.  When the queue length is zero
(the default), a SynchronousQueue is utilized, preserving the previous
appender behavior.  When the queue length is greater than zero, a
bounded queue is utilized, allowing the appender to efficiently drop
logging events when remote receiver (or network) cannot keep up with the
rate of logging events delivered to the appender.