LOGBACK-822: add ServerSocketAppenderBase and SSLServerSocketAppenderBase to core

This pull request moves the server abstractions created for SocketServer to core and refactors them to allow them to be used as the basis for ServerSocketAppenderBase and its SSL-enabled subclass.  Using these base abstractions the implementation of the appender classes is almost trivial.

This request intersects the existing code base in master only in those classes that are part of the as-yet-unreleased SocketServer component.