Driver/receive: Implement audio reorder/jitter buffer (#156)

This PR Introduces a new `VoiceTick` event which collects and reorders all RTP packets to smooth over network instability, as well as to synchronise user audio streams. Raw packet events have been moved to `RtpPacket`, while `SpeakingUpdate`s have been removed as they can be easily computed using the `silent`/`speaking` audio maps included in each event.

Closes #146.

src/driver/tasks/mixer/mod.rs

@@ -657,6 +657,8 @@ impl Mixer {
                     None => {},
                 }
 
+                self.advance_rtp_timestamp();
+
                 return Ok(());
             }
         } else {
@@ -809,6 +811,18 @@ impl Mixer {
         rtp.set_timestamp(rtp.get_timestamp() + MONO_FRAME_SIZE as u32);
     }
 
+    #[inline]
+    // Even if we don't send a packet, we *do* need to keep advancing the timestamp
+    // to make it easier for a receiver to reorder packets and compute jitter measures
+    // wrt. our clock difference vs. theirs.
+    fn advance_rtp_timestamp(&mut self) {
+        let mut rtp = MutableRtpPacket::new(&mut self.packet[..]).expect(
+            "FATAL: Too few bytes in self.packet for RTP header.\
+                (Blame: VOICE_PACKET_MAX?)",
+        );
+        rtp.set_timestamp(rtp.get_timestamp() + MONO_FRAME_SIZE as u32);
+    }
+
     #[inline]
     fn check_and_send_keepalive(&mut self) -> Result<()> {
         if let Some(conn) = self.conn_active.as_mut() {