This reverts commit 6c3620093d.
This commit would cause sender.ReadRTCP() to never return
even when pc associated with this sender was closed.
The aftermath is leaked goroutines that will never stop.
Add an accessor to make getting value easy. Also add
TestPeerConnection_SkipStoppedTransceiver. This commit also cleans
up RTPTransceiver creation. We used a helper function, when we should
have just used the provide constructor