db: Initial trigger support.

[mwkmwkmwk]

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

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