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.WebSocket/DiscordSocketConfig.cs

@@ -126,6 +126,14 @@ namespace Discord.WebSocket
         public bool GuildSubscriptions { get; set; } = true;
 
         /// <summary>
+        ///     Gets or sets the maximum identify concurrency.
+        /// </summary>
+        /// <remarks>
+        ///     This information is provided by Discord.
+        ///     It is only used when using a <see cref="DiscordShardedClient"/> and auto-sharding is disabled.
+        /// </remarks>
+        public int IdentifyMaxConcurrency { get; set; } = 1;
+
         ///     Gets or sets the maximum wait time in milliseconds between GUILD_AVAILABLE events before firing READY.
         ///
         ///     If zero, READY will fire as soon as it is received and all guilds will be unavailable.