Move RTPSender's worker-only state off send_mutex_

supports_bwe_extension_ was guarded by send_mutex_, even though it is
only accessed on the worker thread. Guard it with a new worker_checker_
instead, and DCHECK that the methods that access it run there.
worker_checker_ is detached at construction and binds to the sequence of
the first such call, so RTPSender may still be constructed on another
thread. Methods that only access state shared with the media threads are
not restricted. SupportsPadding() now only takes the lock when a header
extension usable for bandwidth estimation is registered.

Code that uses RTPSender directly must now register header extensions
and generate padding on the same sequence.

Also document the threading model, and remove random_ (only used in the
constructor), unused declarations, obsolete TODOs and meaningless lock
annotations. The GeneratePadding() comment claimed that the lock is held
for the whole method, which is not the case. The RTX payload type map is
now a flat_map.

Bug: webrtc:42223727
Change-Id: I38e51349681ca54f2e4283d001fcae6c48004661
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507260
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48787}