decodebin3: Make update/posting of collection messages atomic

decodebin3: Make update/posting of collection messages atomic

The presence (or not) of a collection on an input will determine whether events
will be throttled so that there are only forwarded when that input gets a valid
collection.

Therefore the input lock should be used.

In addition to that, we want to ensure that the application/user has a chance to
reliably (i.e. synchronously) specify what streams it is interested in by
sending a GST_EVENT_SELECT_STREAMS.

But we cannot allow anything to go forward until that message posting has come
back, otherwise we run in various races.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3872

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7602>