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This also refactors a large portion of the DI toolchain, since
I discovered that you shouldn't be reusing IDbConnection objects.

Instead, most services and stores are now declared transient, and
the webhook cache has been moved to a database-independent storage
singleton by itself.

PluralKit.Bot/Services/WebhookCacheService.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Collections.Concurrent;
+using System.Linq;
+using System.Threading.Tasks;
+using Discord;
+using Discord.WebSocket;
+
+namespace PluralKit.Bot
+{
+    public class WebhookCacheService
+    {
+        public static readonly string WebhookName = "PluralKit Proxy Webhook";
+            
+        private IDiscordClient _client;
+        private ConcurrentDictionary<ulong, Lazy<Task<IWebhook>>> _webhooks;
+
+        public WebhookCacheService(IDiscordClient client)
+        {
+            this._client = client;
+            _webhooks = new ConcurrentDictionary<ulong, Lazy<Task<IWebhook>>>();
+        }
+
+        public async Task<IWebhook> GetWebhook(ulong channelId)
+        {
+            var channel = await _client.GetChannelAsync(channelId) as ITextChannel;
+            if (channel == null) return null;
+            return await GetWebhook(channel);
+        }
+
+        public async Task<IWebhook> GetWebhook(ITextChannel channel)
+        {
+            // We cache the webhook through a Lazy<Task<T>>, this way we make sure to only create one webhook per channel
+            // If the webhook is requested twice before it's actually been found, the Lazy<T> wrapper will stop the
+            // webhook from being created twice.
+            var lazyWebhookValue =
+                _webhooks.GetOrAdd(channel.Id, new Lazy<Task<IWebhook>>(() => GetOrCreateWebhook(channel)));
+            return await lazyWebhookValue.Value;
+        }
+
+        private async Task<IWebhook> GetOrCreateWebhook(ITextChannel channel) =>
+            await FindExistingWebhook(channel) ?? await GetOrCreateWebhook(channel);
+
+        private async Task<IWebhook> FindExistingWebhook(ITextChannel channel) => (await channel.GetWebhooksAsync()).FirstOrDefault(IsWebhookMine);
+        
+        private async Task<IWebhook> DoCreateWebhook(ITextChannel channel) => await channel.CreateWebhookAsync(WebhookName);
+        private bool IsWebhookMine(IWebhook arg) => arg.Creator.Id == _client.CurrentUser.Id && arg.Name == WebhookName;
+    }
+}