Refactor trigger cb to pass isDelete vs duplicate doc on insert

[lejeunerenard]

Previously the trigger callback function signature was:

async function triggerCallback (db, key, record) {}

record was either:

• a duplicate of key for db.insert()
• null for db.delete()

This PR refactors record into isDelete which is true for db.delete() and false for db.insert(). This was motivated by the fact that record didn't give addition information about the document that triggered the callback.