Rework caching

This does a *lot* of things. Essentially, it replaces the existing
individual proxy- and autoproxy caches on the bot end with a
global cache (in Core) that handles all the caching at once, and
automatically invalidates the cache once something changes in the
datastore.

This allows us to do proxying and autoproxying with *zero database
queries* (best-case).

PluralKit.Core/Modules.cs

@@ -3,8 +3,11 @@ using System;
 using App.Metrics;
 
 using Autofac;
+using Autofac.Extensions.DependencyInjection;
 
+using Microsoft.Extensions.Caching.Memory;
 using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
 
 using NodaTime;
 
@@ -23,6 +26,9 @@ namespace PluralKit.Core
             builder.RegisterType<DbConnectionFactory>().AsSelf().SingleInstance();
             builder.RegisterType<PostgresDataStore>().AsSelf().As<IDataStore>();
             builder.RegisterType<SchemaService>().AsSelf();
+            
+            builder.Populate(new ServiceCollection().AddMemoryCache());
+            builder.RegisterType<ProxyCache>().AsSelf().SingleInstance();
         }
     }