Cluster SPI: need to remove references to session in all cases when session is closed.
Cluster SPI: RemoteSession

- TyrusSession.getOpenSessions() now returns only local sessions
- RemoteSessions can be obtained via TyrusSession.getRemoteSessions().
- RemoteSession does NOT implement javax.websocket.Session (it is a subset)
- minor refactoring of TyrusEndpointWrapper