Add denormalised columns to statuses table

This is the second part of the DB denormalisation work. Several columns are added to the statuses table so we can avoid joining tables when looking up which messages can be sent or offered to a contact. This improved query performance in performance tests (see the 545-denormalise-all-the-things branch for comparative tests).

Management of status rows is moved inside the Database interface to ensure the denormalised columns stay consistent.

This can be backported but it will require a migration.