SQLite Driver doesn't maintain revision state persistently

[ncatelli]

The sqlite driver maintains it's state within an in memory representation of the sequence list. This does not match the expected behaviour of the other drivers, which expect the state to be persisted in the database.

// Driver implements github.com/PacketFire/immigrant/pkg/core.Driver, providing
// CRUD methods for operating on a contained sqlite database.
type Driver struct {
	Db        *sql.DB
	Revisions []core.Revision
}

This should be updated to create and maintain a state table.