Receiving RTCP often caused the worker thread to stall for >20 ms
(>100ms observed) due to contention on VideoSender's send_crit_ (used to
protect encoding).
This change removes an unnecessary acquire of send_crit_ and caches
encoder settings in ViEEncoder instead of acquiring them through
::SendCodec() in VCM (which is blocking).
BUG=webrtc:5106
R=stefan@webrtc.org
Review URL: https://codereview.webrtc.org/1433703002 .
Cr-Commit-Position: refs/heads/master@{#10582}
d153a37801