Cache migrated versions list in Migrator and use it to fetch the latest migrated version name [#845 state:resolved]

Also optimized Migrator#current_version class method to fetch
only the latest version number and not all of them.

With this change no matter how many migrations there are the
schema_migrations table is only SELECTed from once.
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>