Adds logging to RTPSender and RTCPSender, pushing an event log pointer from Channel through ModuleRtpRtcpImpl to the Sender objects.
BUG=webrtc:4741
Review URL: https://codereview.webrtc.org/1571283002
Cr-Commit-Position: refs/heads/master@{#11336}
Time to keep old events in buffer is now changeable at runtime.
Added unit test for removing old events from buffer.
Review URL: https://codereview.webrtc.org/1303713002
Cr-Commit-Position: refs/heads/master@{#10302}