aggregator API is inherently racy

The following discussion from gst-plugins-good!711 (merged) should be addressed:

• @meh started a discussion: (+1 comment)

  Hm, what if the pad gets flush stopped and receives a new buffer in the interval? In that case, we may end up not muxing the "best" pad. The alternative solution is to keep a reference to the "best" buffer alongside the best pad, and call drop_buffer() on it once actually processed.

Basically:

• @slomo said:

  The only thing you can do is to pop() and then use up the buffer or queue it up locally. Everything else is racy :) So peek() and drop() are useless API and we just noticed that the aggregator API is bad 🤪