This CL refactors RtpSender and extracts handling of sequence number
assignment and timestamping of padding packets in a separate helper
class.
This is in preparation for allowing deferred sequencing to after the
pacing stage.
Bug: webrtc:11340
Change-Id: I5f8c67f3bb90780b3bdd24afa6ae28dbe9d839a0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208401
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33316}
