This commit changes how we locate the next migration height by including
the scenario where `lnd@v0.15.0` is active. In the new version, we will
see a mixed of new and old logs under the same open channel bucket.
Hence, we need to alter how we locate the next un-migrated height.
This commit adds several utility functions to assist the migration. In
particular, an updateLocator is added to gives us the next un-migration
position in our buckets. This locator helps us to continue the job
in case of an interrupted migration. It also serves as an indicator on
whether the migration is finished or not.