Gateway: Add debug logging around shard handling

Adds some additional logging around some critical sections, rarely hit (i.e., during shard reconnections) in pursuit of issue #69. It's strongly suspected to lie here, at any rate...

src/manager.rs

@@ -26,6 +26,7 @@ use std::sync::Arc;
 use tokio::sync::Mutex;
 #[cfg(feature = "tokio-02-marker")]
 use tokio_compat::sync::Mutex;
+use tracing::debug;
 #[cfg(feature = "twilight")]
 use twilight_gateway::Cluster;
 #[cfg(feature = "twilight")]
@@ -390,15 +391,30 @@ impl Songbird {
 #[async_trait]
 impl VoiceGatewayManager for Songbird {
     async fn initialise(&self, shard_count: u64, user_id: SerenityUser) {
+        debug!(
+            "Initialising Songbird for Serenity: ID {:?}, {} Shards",
+            user_id, shard_count
+        );
         self.initialise_client_data(shard_count, user_id);
+        debug!("Songbird ({:?}) Initialised!", user_id);
     }
 
     async fn register_shard(&self, shard_id: u64, sender: Sender<InterMessage>) {
+        debug!(
+            "Registering Serenity shard handle {} with Songbird",
+            shard_id
+        );
         self.sharder.register_shard_handle(shard_id, sender);
+        debug!("Registered shard handle {}.", shard_id);
     }
 
     async fn deregister_shard(&self, shard_id: u64) {
+        debug!(
+            "Deregistering Serenity shard handle {} with Songbird",
+            shard_id
+        );
         self.sharder.deregister_shard_handle(shard_id);
+        debug!("Deregistered shard handle {}.", shard_id);
     }
 
     async fn server_update(&self, guild_id: SerenityGuild, endpoint: &Option<String>, token: &str) {