netsvc: lazy variant of Agent.setAlarm() that operates after cr.commit

Calling setAlarm among some heavy SQL processing could mean that the
alarm (if set on relatively short period) could fire /before/ the
cursor has been committed.

Using the recent post-commit hooks on the cursor, we can provide a
proper solution to this, asking `setAlarmLater()` to fire after the
commit.

The "cumulative" feature of `setAlarm()` has also been moved to only
the `..Later()` variant.