WIP: rtsp-client: Add session timeout mechanism

The former code left the rtsp-client object remaining after the last session had a timeout, since no teardown request had been received in this case. With this change, an application is allowed to have a timeout mechanism which closes the client object when its last session has timed out. This is done by adding a new return value to gst_rtsp_session_pool_filter functions, which results in a timedout flag being set on the session object. The client object will then close the connection if the flag is set and no more sessions are left.

Fixes #83 (closed)