Add thread guards to JsepTransport

This ensures that JsepTransport's methods are either only accessed on the thread
that creates it, or using methods that are marked for off-thread use
(using a lock to prevent simultaneous access).

The intent is to document the existing contract, and to make it easy to find the
actions needed to convert the class to a pure single-threaded class.

Bug: webrtc:10300
Change-Id: Ib5cdc027632c36baec55179937d6eb664bbaf6f5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/121946
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27427}