When requesting an asset from different threads we had no
guarantee that during the time we lookup an asset (which didn't exist)
and the time we create the asset with the same type/ID another thread
could not end up doing the same thing. In turns we could end up with
2 different threads loading the exact same asset and the cache
basically forgetting about one of the entries meaning that the user
would never get notified about one of those being ready to be used.
There was also the case when requesting "sync" where the user was
requesting an asset while another thread is creating it so it was
still in "ASSET_INITIALIZING" state, meaning that the returned asset
would be NULL which would be considered as an error in apps.
Since the cache lock is recursive we can just take it during the whole
ges_asset_request_async call and have other method still hold it.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5732>
