Add various optimizations and cleanups (#1114)

* Change all Select(... as ...) to OfType

* Add changes according to 194a8aa427

src/Discord.Net.WebSocket/ClientState.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
@@ -72,7 +72,7 @@ namespace Discord.WebSocket
                 switch (channel)
                 {
                     case SocketDMChannel dmChannel:
-                        _dmChannels.TryRemove(dmChannel.Recipient.Id, out var ignored);
+                        _dmChannels.TryRemove(dmChannel.Recipient.Id, out _);
                         break;
                     case SocketGroupChannel groupChannel:
                         _groupChannels.TryRemove(id);