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/retry/strategy.rs

@@ -0,0 +1,84 @@
+use rand::random;
+use std::time::Duration;
+
+/// Logic used to determine how long to wait between retry attempts.
+#[derive(Clone, Copy, Debug, PartialEq)]
+#[non_exhaustive]
+pub enum Strategy {
+    /// The driver will wait for the same amount of time between each retry.
+    Every(Duration),
+    /// Exponential backoff waiting strategy, where the duration between
+    /// attempts (approximately) doubles each time.
+    Backoff(ExponentialBackoff),
+}
+
+impl Strategy {
+    pub(crate) fn retry_in(&self, last_wait: Option<Duration>) -> Duration {
+        match self {
+            Self::Every(t) => *t,
+            Self::Backoff(exp) => exp.retry_in(last_wait),
+        }
+    }
+}
+
+/// Exponential backoff waiting strategy.
+///
+/// Each attempt waits for twice the last delay plus/minus a
+/// random jitter, clamped to a min and max value.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct ExponentialBackoff {
+    /// Minimum amount of time to wait between retries.
+    ///
+    /// *Defaults to 0.25s.*
+    pub min: Duration,
+    /// Maximum amount of time to wait between retries.
+    ///
+    /// This will be clamped to `>=` min.
+    ///
+    /// *Defaults to 10s.*
+    pub max: Duration,
+    /// Amount of uniform random jitter to apply to generated wait times.
+    /// I.e., 0.1 will add +/-10% to generated intervals.
+    ///
+    /// This is restricted to within +/-100%.
+    ///
+    /// *Defaults to `0.1`.*
+    pub jitter: f32,
+}
+
+impl Default for ExponentialBackoff {
+    fn default() -> Self {
+        Self {
+            min: Duration::from_millis(250),
+            max: Duration::from_secs(10),
+            jitter: 0.1,
+        }
+    }
+}
+
+impl ExponentialBackoff {
+    pub(crate) fn retry_in(&self, last_wait: Option<Duration>) -> Duration {
+        let attempt = last_wait.map(|t| 2 * t).unwrap_or(self.min);
+        let perturb = (1.0 - (self.jitter * 2.0 * (random::<f32>() - 1.0)))
+            .max(0.0)
+            .min(2.0);
+        let mut target_time = attempt.mul_f32(perturb);
+
+        // Now clamp target time into given range.
+        let safe_max = if self.max < self.min {
+            self.min
+        } else {
+            self.max
+        };
+
+        if target_time > safe_max {
+            target_time = safe_max;
+        }
+
+        if target_time < self.min {
+            target_time = self.min;
+        }
+
+        target_time
+    }
+}