DB IDO: Only update 'next_check' column when manually scheduling a check

DB IDO: Only update 'next_check' column when manually scheduling a check

Otherwise the changes from #7287 already take care of setting
the proper next check time from inside ProcessCheckResult().

There is no need to use the generic OnNextCheckChanged signal
but instead we're using a new one, locally just for DB IDO.

fixes #11019