Prevent concurrent access to AudioSendStream's configuration.

By design:
 * OnPacketAdded() is meant to be called on pacer thread.
 * Reconfigure()   is meant to be called on worker thread.
Thus we guard against race condition on config_ member.

Possible downside: packet filtering based on ssrc might be slowed down.

Bug: webrtc:9849
Change-Id: I734bb9b34b01db160705897adb1b58e866e12639
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146980
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28691}