feature: Implement gateway ratelimit (#1537)

* Implement gateway ratelimit

* Remove unused code

* Share WebSocketRequestQueue between clients

* Add global limit and a way to change gateway limits

* Refactoring variable to fit lib standards

* Update xml docs

* Update xml docs

* Move warning to remarks

* Remove specific RequestQueue for WebSocket and other changes

The only account limit is for identify that is dealt in a different way (exclusive semaphore), so websocket queues can be shared with REST and don't need to be shared between clients anymore.

Also added the ratelimit for presence updates.

* Add summary to IdentifySemaphoreName

* Fix spacing

* Add max_concurrency and other fixes

- Add session_start_limit to GetBotGatewayResponse
- Add GetBotGatewayAsync to IDiscordClient
- Add master/slave semaphores to enable concurrency
- Not store semaphore name as static
- Clone GatewayLimits when cloning the Config

* Add missing RequestQueue parameter and wrong nullable

* Add RequeueQueue paramater to Webhook

* Better xml documentation

* Remove GatewayLimits class and other changes

- Remove GatewayLimits
- Transfer a few properties to DiscordSocketConfig
- Remove unnecessary usings

* Remove unnecessary using and wording

* Remove more unnecessary usings

* Change named Semaphores to SemaphoreSlim

* Remove unused using

* Update branch

* Fix merge conflicts and update to new ratelimit

* Fixing merge, ignore limit for heartbeat, and dispose

* Missed one place and better xml docs.

* Wait identify before opening the connection

* Only request identify ticket when needed

* Move identify control to sharded client

* Better description for IdentifyMaxConcurrency

* Add lock to InvalidSession

src/Discord.Net.Core/IDiscordClient.cs

@@ -274,5 +274,15 @@ namespace Discord
         ///     that represents the number of shards that should be used with this account.
         /// </returns>
         Task<int> GetRecommendedShardCountAsync(RequestOptions options = null);
+
+        /// <summary>
+        ///     Gets the gateway information related to the bot.
+        /// </summary>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <returns>
+        ///     A task that represents the asynchronous get operation. The task result contains a <see cref="BotGateway"/>
+        ///     that represents the gateway information related to the bot.
+        /// </returns>
+        Task<BotGateway> GetBotGatewayAsync(RequestOptions options = null);
     }
 }