Client: keep a ref to the session

Client: keep a ref to the session

Don't just keep a weak ref to the session objects but use a hard ref. We
will be notified when a session is removed from the pool (expired) with
the new session-removed signal.
Don't automatically close the RTSP connection when all the sessions of
a client are removed, a client can continue to operate and it can create
a new session if it wants. If you want to remove the client from the
server, you have to use gst_rtsp_server_client_filter() now.

Based on patch from Ognyan Tonchev <ognyan.tonchev at axis.com>

See https://bugzilla.gnome.org/show_bug.cgi?id=732226