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/tasks/error.rs

@@ -17,6 +17,7 @@ pub enum Recipient {
 pub type Result<T> = std::result::Result<T, Error>;
 
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     Crypto(CryptoError),
     /// Received an illegal voice packet on the voice UDP socket.

src/driver/tasks/mixer.rs

@@ -1,7 +1,8 @@
-use super::{disposal, error::Result, message::*, Config};
+use super::{disposal, error::Result, message::*};
 use crate::{
     constants::*,
     tracks::{PlayMode, Track},
+    Config,
 };
 use audiopus::{
     coder::Encoder as OpusEncoder,

src/driver/tasks/mod.rs

@@ -9,11 +9,8 @@ pub(crate) mod udp_rx;
 pub(crate) mod udp_tx;
 pub(crate) mod ws;
 
-use super::{
-    connection::{error::Error as ConnectionError, Connection},
-    Config,
-};
-use crate::events::CoreContext;
+use super::connection::{error::Error as ConnectionError, Connection};
+use crate::{events::CoreContext, Config};
 use flume::{Receiver, RecvError, Sender};
 use message::*;
 #[cfg(not(feature = "tokio-02-marker"))]

src/driver/tasks/udp_rx.rs

@@ -1,12 +1,9 @@
 use super::{
     error::{Error, Result},
     message::*,
+    Config,
 };
-use crate::{
-    constants::*,
-    driver::{Config, DecodeMode},
-    events::CoreContext,
-};
+use crate::{constants::*, driver::DecodeMode, events::CoreContext};
 use audiopus::{
     coder::Decoder as OpusDecoder,
     error::{Error as OpusError, ErrorCode},