Store buffer after creating it. (#4186)

* Refactor receiver and buffer into Base and higher layer.

To be able to share code/functionality with relay.

* WIP

* WIP

* WIP

* WIP

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* WIP

* WIP

* clean up

* deps

* fix test

* fix test

* Store buffer after creating it.

Also changing signature of creator function as it could call TrackInfo()
and get into a deadlock.

* fix double unlock

* add some more debug logging

pkg/sfu/buffer/buffer.go

@@ -311,6 +311,11 @@ func (b *Buffer) getOnClose() func() {
 
 func (b *Buffer) sendPLI() {
 	ssrc := b.BufferBase.SSRC()
+	if ssrc == 0 {
+		return
+	}
+
+	b.logger.Debugw("send pli", "mediaSSRC", ssrc)
 	pli := []rtcp.Packet{
 		&rtcp.PictureLossIndication{
 			SenderSSRC: ssrc,