Lazy loading migrations

In our project FluentMigrator used for about 2 years. During this time in the assembly with migrations accumulated 1748 migrations. Because of this, the initial initialization migration was large (15-16 seconds). This happens because when the assembly with migrations creates instances of all migrations (even if you need to install only 1-2 migrations or do not need any). Such changes in our copy FluentMigrator reduced load time to 20-30ms.