Gateway: Add connection timeout, add Config to gateway. (#51)

This change fixes tasks hanging due to rare cases of messages being lost between full Discord reconnections by placing a configurable timeout on the `ConnectionInfo` responses. This is a companion fix to [serenity#1255](https://github.com/serenity-rs/serenity/pull/1255). To make this doable, `Config`s are now used by all versions of `Songbird`/`Call`, and relevant functions are  added to simplify setup with configuration. These are now non-exhaustive, correcting an earlier oversight. For future extensibility, this PR moves the return type of `join`/`join_gateway` into a custom future (no longer leaking flume's `RecvFut` type).

Additionally, this fixes the Makefile's feature sets for driver/gateway-only compilation.

This is a breaking change in:
* the return types of `join`/`join_gateway`
* moving `crate::driver::Config` -> `crate::Config`,
* `Config` and `JoinError` becoming `#[non_breaking]`.

This was tested via `cargo make ready`, and by testing `examples/serenity/voice_receive` with various timeout settings.

src/config.rs

@@ -0,0 +1,116 @@
+#[cfg(feature = "driver-core")]
+use super::driver::{CryptoMode, DecodeMode};
+
+#[cfg(feature = "gateway-core")]
+use std::time::Duration;
+
+/// Configuration for drivers and calls.
+#[derive(Clone, Debug)]
+#[non_exhaustive]
+pub struct Config {
+    #[cfg(feature = "driver-core")]
+    /// Selected tagging mode for voice packet encryption.
+    ///
+    /// Defaults to [`CryptoMode::Normal`].
+    ///
+    /// Changes to this field will not immediately apply if the
+    /// driver is actively connected, but will apply to subsequent
+    /// sessions.
+    ///
+    /// [`CryptoMode::Normal`]: CryptoMode::Normal
+    pub crypto_mode: CryptoMode,
+    #[cfg(feature = "driver-core")]
+    /// Configures whether decoding and decryption occur for all received packets.
+    ///
+    /// If voice receiving voice packets, generally you should choose [`DecodeMode::Decode`].
+    /// [`DecodeMode::Decrypt`] is intended for users running their own selective decoding,
+    /// who rely upon [user speaking events], or who need to inspect Opus packets.
+    /// If you're certain you will never need any RT(C)P events, then consider [`DecodeMode::Pass`].
+    ///
+    /// Defaults to [`DecodeMode::Decrypt`]. This is due to per-packet decoding costs,
+    /// which most users will not want to pay, but allowing speaking events which are commonly used.
+    ///
+    /// [`DecodeMode::Decode`]: DecodeMode::Decode
+    /// [`DecodeMode::Decrypt`]: DecodeMode::Decrypt
+    /// [`DecodeMode::Pass`]: DecodeMode::Pass
+    /// [user speaking events]: crate::events::CoreEvent::SpeakingUpdate
+    pub decode_mode: DecodeMode,
+    #[cfg(feature = "gateway-core")]
+    /// Configures the amount of time to wait for Discord to reply with connection information
+    /// if [`Call::join`]/[`join_gateway`] are used.
+    ///
+    /// This is a useful fallback in the event that:
+    ///  * the underlying Discord client restarts and loses a join request, or
+    ///  * a channel join fails because the bot is already believed to be there.
+    ///
+    /// Defaults to 10 seconds. If set to `None`, connections will never time out.
+    ///
+    /// [`Call::join`]: crate::Call::join
+    /// [`join_gateway`]: crate::Call::join_gateway
+    pub gateway_timeout: Option<Duration>,
+    #[cfg(feature = "driver-core")]
+    /// Number of concurrently active tracks to allocate memory for.
+    ///
+    /// This should be set at, or just above, the maximum number of tracks
+    /// you expect your bot will play at the same time. Exceeding the size of
+    /// the internal queue will trigger a larger memory allocation and copy,
+    /// possibly causing the mixer thread to miss a packet deadline.
+    ///
+    /// Defaults to `1`.
+    ///
+    /// Changes to this field in a running driver will only ever increase
+    /// the capacity of the track store.
+    pub preallocated_tracks: usize,
+}
+
+impl Default for Config {
+    fn default() -> Self {
+        Self {
+            #[cfg(feature = "driver-core")]
+            crypto_mode: CryptoMode::Normal,
+            #[cfg(feature = "driver-core")]
+            decode_mode: DecodeMode::Decrypt,
+            #[cfg(feature = "gateway-core")]
+            gateway_timeout: Some(Duration::from_secs(10)),
+            #[cfg(feature = "driver-core")]
+            preallocated_tracks: 1,
+        }
+    }
+}
+
+#[cfg(feature = "driver-core")]
+impl Config {
+    /// Sets this `Config`'s chosen cryptographic tagging scheme.
+    pub fn crypto_mode(mut self, crypto_mode: CryptoMode) -> Self {
+        self.crypto_mode = crypto_mode;
+        self
+    }
+
+    /// Sets this `Config`'s received packet decryption/decoding behaviour.
+    pub fn decode_mode(mut self, decode_mode: DecodeMode) -> Self {
+        self.decode_mode = decode_mode;
+        self
+    }
+
+    /// Sets this `Config`'s number of tracks to preallocate.
+    pub fn preallocated_tracks(mut self, preallocated_tracks: usize) -> Self {
+        self.preallocated_tracks = preallocated_tracks;
+        self
+    }
+
+    /// This is used to prevent changes which would invalidate the current session.
+    pub(crate) fn make_safe(&mut self, previous: &Config, connected: bool) {
+        if connected {
+            self.crypto_mode = previous.crypto_mode;
+        }
+    }
+}
+
+#[cfg(feature = "gateway-core")]
+impl Config {
+    /// Sets this `Config`'s timeout for joining a voice channel.
+    pub fn gateway_timeout(mut self, gateway_timeout: Option<Duration>) -> Self {
+        self.gateway_timeout = gateway_timeout;
+        self
+    }
+}