Added a db migrate module.

Added a db migrate module.

The migrate module is for migrating instead of resetting the db between changes.

This commit also changes the db version twice;

7->8: dbVersion is now kept as a per-realm value, as all other data is stored that way. If it would be kept global it would mean migration would only run on on realm, then increase the dbVersion globally, making other realm's data out of sync.

8->9: Added "links" back from a groupParse to the encounter it was for.