db: Initial trigger support.

Working:

- database model
- adding and deleting triggers (in backend and tracker)
- marking triggers as processed (in backend only, not supported in
  the tracker yet)
- busting processed triggers back to pending state on all check
  types except bindata and list (in backend and tracker)
- getting a list of pending triggers from the backend

Not yet implemented:

- keeping track of triggers deleted during a transaction (so that they can
  be removed from the work queue + bindata/list check managers)
- busting triggers on bindata and list checks
- trigger work queue and distribution to plugins
- tests