Driver: Automate (re)connection logic (#81)

This PR adds several enhancements to Driver connection logic:
* Driver (re)connection attempts now have a default timeout of around 10s.
* The driver will now attempt to retry full connection attempts using a user-provided strategy: currently, this defaults to 5 attempts under an exponential backoff strategy.
* The driver will now fire `DriverDisconnect` events at the end of any session -- this unifies (re)connection failure events with session expiry as seen in #76, which should provide users with enough detail to know *which* voice channel to reconnect to. Users still need to be careful to read the session/channel IDs to ensure that they aren't overwriting another join.

This has been tested using `cargo make ready`, and by setting low timeouts to force failures in the voice receive example (with some additional error handlers).

Closes #68.

src/driver/tasks/message/core.rs

@@ -2,7 +2,7 @@
 
 use crate::{
     driver::{connection::error::Error, Bitrate, Config},
-    events::EventData,
+    events::{context_data::DisconnectReason, EventData},
     tracks::Track,
     ConnectionInfo,
 };
@@ -12,6 +12,8 @@ use flume::Sender;
 #[derive(Debug)]
 pub enum CoreMessage {
     ConnectWithResult(ConnectionInfo, Sender<Result<(), Error>>),
+    RetryConnect(usize),
+    SignalWsClosure(usize, ConnectionInfo, Option<DisconnectReason>),
     Disconnect,
     SetTrack(Option<Track>),
     AddTrack(Track),