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.

examples/serenity/voice_receive/src/main.rs

@@ -28,14 +28,14 @@ use serenity::{
 };
 
 use songbird::{
-    driver::{Config as DriverConfig, DecodeMode},
+    driver::DecodeMode,
     model::payload::{ClientConnect, ClientDisconnect, Speaking},
+    Config,
     CoreEvent,
     Event,
     EventContext,
     EventHandler as VoiceEventHandler,
     SerenityInit,
-    Songbird,
 };
 
 struct Handler;
@@ -167,16 +167,13 @@ async fn main() {
     // Here, we need to configure Songbird to decode all incoming voice packets.
     // If you want, you can do this on a per-call basis---here, we need it to
     // read the audio data that other people are sending us!
-    let songbird = Songbird::serenity();
-    songbird.set_config(
-        DriverConfig::default()
-            .decode_mode(DecodeMode::Decode)
-    );
+    let songbird_config = Config::default()
+        .decode_mode(DecodeMode::Decode);
 
     let mut client = Client::builder(&token)
         .event_handler(Handler)
         .framework(framework)
-        .register_songbird_with(songbird.into())
+        .register_songbird_from_config(songbird_config)
         .await
         .expect("Err creating client");