The ALTER TABLE operations previously used were causing the DB to die when run on the production TagManifest table which has 7 million rows. We instead now use new mapping tables, which is less nice, but these are temporary anyway, so hopefully we only have to deal with their ugliness for a short duration.
This change also starts passing in the manifest interface, rather than the raw data, to the model for writing.
Note that this change does *not* backfill the existing rows in to the new tables; that will occur in a followup PR. The new columns in `tagmanifest` and `tagmanifestlabel` will be used to track the backfill, as it will occur in a worker.
