feature: Add cache purging methods (#1478)

src/Discord.Net.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -623,6 +623,13 @@ namespace Discord.WebSocket
                 return state.RemoveChannel(id) as SocketGuildChannel;
             return null;
         }
+        internal void PurgeChannelCache(ClientState state)
+        {
+            foreach (var channelId in _channels)
+                state.RemoveChannel(channelId);
+
+            _channels.Clear();
+        }
 
         //Voice Regions
         /// <summary>
@@ -797,6 +804,21 @@ namespace Discord.WebSocket
             }
             return null;
         }
+        internal void PurgeGuildUserCache()
+        {
+            var members = Users;
+            var self = CurrentUser;
+            _members.Clear();
+            _members.TryAdd(self.Id, self);
+
+            DownloadedMemberCount = _members.Count;
+
+            foreach (var member in members)
+            {
+                if (member.Id != self.Id)
+                    member.GlobalUser.RemoveRef(Discord);
+            }
+        }
 
         /// <inheritdoc />
         public async Task DownloadUsersAsync()