Cache info about users, cleanup for module

Core/Startup.cs

@@ -1,4 +1,5 @@
 using BotFramework;
+using Kruzya.TelegramBot.Core.Cache;
 using Kruzya.TelegramBot.Core.Data;
 using Kruzya.TelegramBot.Core.Extensions;
 using Microsoft.AspNetCore.Builder;
@@ -7,6 +8,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
+using Telegram.Bot.Types;
 
 namespace Kruzya.TelegramBot.Core
 {
@@ -36,6 +38,9 @@ namespace Kruzya.TelegramBot.Core
             services.AddDbContext<CoreContext>(ctx =>
                 ctx.UseMySql(connectionString, ServerVersion.AutoDetect(connectionString)));
 
+            // Cache for users.
+            services.AddSingleton<ICacheStorage<long, User>, MemoryCache<long, User>>();
+
             // Trigger module handlers.
             _core.Modules.ConfigureServices(services);
         }