Revert "playbin2: Only allow to set the URIs in states <= READY or from an about-to-finish signal handler"

This reverts commit 7335ce5d.

Support abusing the uri property to configure the next uri to play
outside of the about-to-finish handler for the time being after all.
We also shouldn't use thread private structures for this, since it
should be possible to block the thread that emitted about-to-finish
while the main thread sets the uri property. See #607226.