hlsdemux2: Handle loss of synchronization in live

hlsdemux2: Handle loss of synchronization in live

Loss of synchronization happens when the updated media playlist has no
relationship to the previous ones. This could happen because of network issues,
server issues, etc...

When this happens, we take no chance and "reset" ourselves so that we can "seek
back to live" against the new updated playlists.

Since this happens at the "media playlist update" level, make sure the custom
flow return is propagated up.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2839>