Allow for dynamic session addition in AbstractSocketInitiator

Right now you can have multiple sessions in same SocketInitiator only when you configure them at startup.
Would be nice to have a programmatic way to add new sessions to socket initiator dynamically, either via code an application level or via some JMX adapter.

It's possible to work around this by creating a new SocketIntiator, but that may create threading issues for people using single-threaded strategies and having single-threaded assumptions in their application code.

Not sure if the right way is to provide a AbstractSocketInitiator.addSession() (and removeSession) function, or if there's a better approach.