Added insert batching to DB init
Added insert batching to DB init

refs https://github.com/TryGhost/knex-migrator/commit/8d9a561cab0b7b7f5745dd36bcfb7a06568d2311

- in the referenced commit I mentioned a further optimization to reduce
  the number of queries down
- this commits uses knex's `batchInsert` function to insert the data in
  batches to reduce the overhead of many individual queries
- in Ghost, these two commits takes the "add migrations" during the init
  from ~140ms to ~10ms

TryGhost/knex-migrator