Voice Rework -- Events, Track Queues (#806)

This implements a proof-of-concept for an improved audio frontend. The largest change is the introduction of events and event handling: both by time elapsed and by track events, such as ending or looping. Following on from this, the library now includes a basic, event-driven track queue system (which people seem to ask for unusually often). A new sample, `examples/13_voice_events`, demonstrates both the `TrackQueue` system and some basic events via the `~queue` and `~play_fade` commands.

Locks are removed from around the control of `Audio` objects, which should allow the backend to be moved to a more granular futures-based backend solution in a cleaner way.

src/driver/tasks/udp_tx.rs

@@ -0,0 +1,45 @@
+use super::message::*;
+use crate::constants::*;
+use discortp::discord::MutableKeepalivePacket;
+use flume::Receiver;
+use tokio::{
+    net::udp::SendHalf,
+    time::{timeout_at, Elapsed, Instant},
+};
+use tracing::{error, info, instrument, trace};
+
+#[instrument(skip(udp_msg_rx))]
+pub(crate) async fn runner(udp_msg_rx: Receiver<UdpTxMessage>, ssrc: u32, mut udp_tx: SendHalf) {
+    info!("UDP transmit handle started.");
+
+    let mut keepalive_bytes = [0u8; MutableKeepalivePacket::minimum_packet_size()];
+    let mut ka = MutableKeepalivePacket::new(&mut keepalive_bytes[..])
+        .expect("FATAL: Insufficient bytes given to keepalive packet.");
+    ka.set_ssrc(ssrc);
+
+    let mut ka_time = Instant::now() + UDP_KEEPALIVE_GAP;
+
+    loop {
+        use UdpTxMessage::*;
+        match timeout_at(ka_time, udp_msg_rx.recv_async()).await {
+            Err(Elapsed { .. }) => {
+                trace!("Sending UDP Keepalive.");
+                if let Err(e) = udp_tx.send(&keepalive_bytes[..]).await {
+                    error!("Fatal UDP keepalive send error: {:?}.", e);
+                    break;
+                }
+                ka_time += UDP_KEEPALIVE_GAP;
+            },
+            Ok(Ok(Packet(p))) =>
+                if let Err(e) = udp_tx.send(&p[..]).await {
+                    error!("Fatal UDP packet send error: {:?}.", e);
+                    break;
+                },
+            Ok(Err(_)) | Ok(Ok(Poison)) => {
+                break;
+            },
+        }
+    }
+
+    info!("UDP transmit handle stopped.");
+}