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/video_coding_impl.cc

@@ -310,9 +310,4 @@ VideoCodingModule* VideoCodingModule::Create(Clock* clock,
                                    nullptr);
 }
 
-void VideoCodingModule::Destroy(VideoCodingModule* module) {
-  if (module != NULL) {
-    delete static_cast<VideoCodingModuleImpl*>(module);
-  }
-}
 }  // namespace webrtc