Fix racy pending frame status in SimulcastEncoderAdapter.

While each sub-encoder (in a StreamContext) can be assumed to process
frames in order, there is the chance for a data race since the
`OnEncodedImage` callback can happen asynchronously from another thread.
This CL makes sure to protect the `pending_rtp_timestamps_` list using a
mutex to prevent issue in such cases.

A pretty complex unit test is also added, that is consistely able to
reproduce this problem when executed under TSAN.

Bug: webrtc:467444018
Change-Id: I595dd403558549b338ac6a67ebc87f6cb610a481
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/457620
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47186}