Untangle ViEChannel and ViEEncoder.

Extracts shared members outside the two objects, removing PayloadRouter
from receivers and the VCM for ViEChannel from senders.

Removes Start/StopThreadsAndSetSharedMembers that was used to set the
shared state between them.

Also adding DCHECKs to document what's only used by the
sender/receiver side.

BUG=webrtc:5494
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1654913002 .

Cr-Commit-Position: refs/heads/master@{#11500}

webrtc/modules/video_coding/include/video_coding.h

@@ -75,8 +75,6 @@ class VideoCodingModule : public Module {
 
   static VideoCodingModule* Create(Clock* clock, EventFactory* event_factory);
 
-  static void Destroy(VideoCodingModule* module);
-
   // Get supported codec settings using codec type
   //
   // Input: