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/driver/mod.rs

@@ -11,13 +11,11 @@
 #[cfg(feature = "internals")]
 pub mod bench_internals;
 
-mod config;
 pub(crate) mod connection;
 mod crypto;
 mod decode_mode;
 pub(crate) mod tasks;
 
-pub use config::Config;
 use connection::error::{Error, Result};
 pub use crypto::CryptoMode;
 pub(crate) use crypto::CryptoState;
@@ -29,6 +27,7 @@ use crate::{
     events::EventData,
     input::Input,
     tracks::{self, Track, TrackHandle},
+    Config,
     ConnectionInfo,
     Event,
     EventHandler,
@@ -212,13 +211,19 @@ impl Driver {
         self.send(CoreMessage::SetTrack(None))
     }
 
-    /// Sets the configuration for this driver.
+    /// Sets the configuration for this driver (and parent `Call`, if applicable).
     #[instrument(skip(self))]
     pub fn set_config(&mut self, config: Config) {
         self.config = config.clone();
         self.send(CoreMessage::SetConfig(config))
     }
 
+    /// Returns a view of this driver's configuration.
+    #[instrument(skip(self))]
+    pub fn config(&self) -> &Config {
+        &self.config
+    }
+
     /// Attach a global event handler to an audio context. Global events may receive
     /// any [`EventContext`].
     ///