VideoRtpReceiver & AudioRtpReceiver threading fixes.

For implementations where the signaling and worker threads are not
the same thread, this significantly cuts down on Thread::Invoke()s that
would block the signaling thread while waiting for the worker thread.

For Audio and Video Rtp receivers, the following methods now do not
block the signaling thread:
* GetParameters
* SetJitterBufferMinimumDelay
* GetSources
* SetFrameDecryptor / GetFrameDecryptor
* SetDepacketizerToDecoderFrameTransformer

Importantly this change also makes the track() accessor accessible
directly from the application thread (bypassing the proxy) since
for receiver objects, the track object is const.

Other changes:

* Remove RefCountedObject inheritance, use make_ref_counted instead.
* Every member variable in the rtp receiver classes is now RTC_GUARDED
* Stop() now fully clears up worker thread state, and Stop() is
  consistently called before destruction. This means that there's one
  thread hop instead of at least 4 before (sometimes more), per receiver.
* OnChanged triggered volume for audio tracks is done asynchronously.
* Deleted most of the JitterBufferDelay implementation. Turns out that
  it was largely unnecessary overhead and complexity.

It seems that these two classes are copy/pasted to a large extent
so further refactoring would be good in the future, as to not have to
fix each issue twice.

Bug: chromium:1184611
Change-Id: I1ba5c3abbd1b0571f7d12850d64004fd2d83e5e2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218605
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34022}

pc/rtp_transceiver_unittest.cc

@@ -93,6 +93,7 @@ class RtpTransceiverUnifiedPlanTest : public ::testing::Test {
                          rtc::Thread::Current(),
                          sender_),
                      RtpReceiverProxyWithInternal<RtpReceiverInternal>::Create(
+                         rtc::Thread::Current(),
                          rtc::Thread::Current(),
                          receiver_),
                      channel_manager_.get(),
@@ -162,6 +163,7 @@ class RtpTransceiverTestForHeaderExtensions : public ::testing::Test {
                          rtc::Thread::Current(),
                          sender_),
                      RtpReceiverProxyWithInternal<RtpReceiverInternal>::Create(
+                         rtc::Thread::Current(),
                          rtc::Thread::Current(),
                          receiver_),
                      channel_manager_.get(),