To avoid race conditions with gst_task_stop(); gst_task_join() with
another thread doing gst_task_pause(), the joining thread would be
waiting for the task to stop but it would never happen. So just
use gst_task_stop() everywhere to prevent more mutexes
Check if the stream is live before checking if it is EOS as a live
stream might be considered EOS when it just needs to wait for a manifest
update to proceed with the next fragments
During flushing seeks the flushing flow return will propagate up to the
source element and all pads are going to have the flushing flag set.
So before restarting also remove that flag together with the EOS one.
We don't do that when pushing the flush stop event because our event
handler for the proxypad will drop all events.
