decodebin3: Release selection lock when pushing EOS
We can't keep the lock otherwise this would lock other actions. In order to keep
it safe, we grab a list of peer pads to send EOS to with the lock taken, then
send to the peer pads with the lock released.
Also make sure the selection lock is taken for another call to this function
Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/847
Change-Id: I8c811ff53b77ed62b79a3e90f1f4dab616f02e36
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/971>