Replay out of sequence migrations

Now that the schema file is a thing of the past, we should be able to reply migration files out of sequence. This DOES NOT mean out of order.

Basically, the thing this is trying to fix is if your schema is in some sort of version control, such as git, and developers create migration files in two separate branches. As long as there are no collisions or conflicts, this should be perfectly OK to do. What we will be able to do here is scan the schema_info table and find out which migrations are missing and replay them individually. This means that if a migration gets missed, for example because it's version is earlier than a migration from another branch but that branch is merged into a master branch first, we will be able to reply it and catch up.

The caveat here is that if a snapshot is being created, the database still has to be in the last known latest version. There may also be problems if migrations refer to the same elements, but even this will be fairly fault tollerant.