rtmp2/connection: Separate inner from outer cancelling

The connection cancels itself when it is closed. To avoid the
cancellable passed to `gst_rtmp_connection_new` from being unexpectedly
cancelled, separate inner from outer cancellation by holding two
cancellables.

Fixes gstreamer/gst-plugins-bad#1558

Part-of: <gstreamer/gst-plugins-bad!2119>
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merge request