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

Fixes gstreamer/gst-plugins-bad#1558

Part-of: <gstreamer/gst-plugins-bad!2119>
29 jobs for !2119 with backport-mr-2111-into-1.18-rtmp2-inner-cancel in 25 minutes and 42 seconds (queued for 4 seconds)
merge request