decodebin3: Protect fields related to streams handling with the SELECTION_LOCK

decodebin3: Protect fields related to streams handling with the SELECTION_LOCK

Fields related to stream handling (input_streams,
output_streams, slots, guint slot_id) where used totally unprotected
until know.

This lead to several races, especially playing back RTSP streams.

To protect those fields, the OBJECT_LOCK can not be used as we sometimes
need to be able to post message on the bus while holding it.

decodebin3 already has a lock to manage stream selection, and in the end
it makes sense to protect all the stream management fields with the same
lock which is why we reuse the SELECTION_LOCK here.

https://bugzilla.gnome.org/show_bug.cgi?id=784012