LOGBACK-824: add ServerSocketAppender classes to access

SSLServerSocketAppender has an 'ssl' property of type SSLConfiguration.

LOGBACK-823: add ServerSocketAppender classes to classic

SSLServerSocketAppender has an "ssl" property of type SSLConfiguration.

Duh.

Duh.

LOGBACK-822: add ServerSocketAppenderBase and SSLServerSocketAppenderBase to core

These classes are all part of the implementation and should not be
visible outside of the package.

The ObjectInputStream reference (oos) was being closed without checking
to see if it was ever created.  If the ObjectInputStream constructor
happened to throw an exception, we'd have an NPE here.  Not likely, but
worth handling properly.

This was done to be consistent with the implementation in
ServerSocketAppenderBase, but isn't strictly required, as all concurrent
tasks for this server will be blocked in I/O calls, not thread waits.

This ensures that any clients that are blocked waiting to take the next
event from the queue will shut down cleanly.

This will allow the created ExecutorService to be shutdown in the
relevant stop method.

In the existing RemoteAppenderStreamClient, an InputStream obtained from
the underlying socket was passed in the constructor.  While the client
implementation closed the stream, it did not close the socket.  Now the
socket is passed to the client to ensure the socket gets closed.

Implemented lifecyle and appender methods, and added unit tests and
supporting classes.

Previously only the queue size was exposed.  Having the queue itself
exposed for configuration makes it easier to write a unit test that
isn't overly complicated.

The stop method was iterating a copy of the collection.  Since this is
exactly what accept(ClientVisitor) does, stop can simply delegate to
accept with a visitor that closes each visited client.

This method will allow a user of a ServerRunner to visit each of the
connected clients in order to do something with the client.