Gateway: Twilight 0.15 support (#171)

This patch changes around quite a few things.
The main entrypoint for twilight besides process will now be the
TwilightMap which concists of command senders for each shard.

This simplifies parts of the code as there is not any difference
between shards and clusters anymore.

src/driver/mod.rs

@@ -30,6 +30,8 @@ pub use mix_mode::MixMode;
 #[cfg(test)]
 pub use test_config::*;
 
+#[cfg(feature = "builtin-queue")]
+use crate::tracks;
 #[cfg(feature = "builtin-queue")]
 use crate::tracks::TrackQueue;
 use crate::{