Call ConnectionService's registerChannel directly.
Replace calls to Session.request(Buffer)Buffer with calls to ConnectionService.request.

The underlying code is very ConnectionService specific.
Move getX11ForwardingAcceptorFactory to common.FactoryManager.

There's nothing in the RFC's to say this is server only; just suggestions for other
related services that they SHOULD be server only.
Drop X11ForwardSupport parameter from ConnectionService constructor.

Support can be detected by checking the factory manager for the presence
of an X11ForwardingAcceptorFactory.
Create AgentForwardSupport in ConnectionService (when supported).

Also pass it the ConnectionService and not the Session.
Delete registerChannel and unregisterChannel from Session; use ConnectionService.
Make findService hack local to ClientSessionImpl.