convert Tracks into Medias and Formats (#155)

* split tracks from medias

* move tracks into dedicated package

* move media into dedicated package

* edit Medias.Marshal() in order to return SDP

* add medias.Find() and simplify examples

* improve coverage

* fix rebase errors

* replace TrackIDs with MediaIDs

* implement media-specific and track-specific callbacks for reading RTCP and RTP packets

* rename publish into record, read into play

* add v2 tag

* rename tracks into formats

serverwriter.go

@@ -0,0 +1,45 @@
+package gortsplib
+
+import (
+	"github.com/aler9/gortsplib/v2/pkg/ringbuffer"
+)
+
+type serverWriter struct {
+	running bool
+	buffer  *ringbuffer.RingBuffer
+
+	done chan struct{}
+}
+
+func (sw *serverWriter) start() {
+	if !sw.running {
+		sw.running = true
+		sw.done = make(chan struct{})
+		go sw.run()
+	}
+}
+
+func (sw *serverWriter) stop() {
+	if sw.running {
+		sw.buffer.Close()
+		<-sw.done
+		sw.running = false
+	}
+}
+
+func (sw *serverWriter) run() {
+	defer close(sw.done)
+
+	for {
+		tmp, ok := sw.buffer.Pull()
+		if !ok {
+			return
+		}
+
+		tmp.(func())()
+	}
+}
+
+func (sw *serverWriter) queue(cb func()) {
+	sw.buffer.Push(cb)
+}