feat: async cache

this breaks logging bot permissions to Sentry.

we haven't had a need to check those recently (permissions issues were because of broken cache), so this is fine for now
this should be re-added in the future though

PluralKit.Bot/Services/PeriodicStatCollector.cs

@@ -52,7 +52,7 @@ namespace PluralKit.Bot
             await foreach (var guild in _cache.GetAllGuilds())
             {
                 guildCount++;
-                foreach (var channel in _cache.GetGuildChannels(guild.Id))
+                foreach (var channel in await _cache.GetGuildChannels(guild.Id))
                 {
                     if (DiscordUtils.IsValidGuildChannel(channel))
                         channelCount++;