This commit introduces a new kinda special `OnRescheduleCheck` signal
that is emitted whenever we want to inform the checker to reschedule the
checkable at a specific timestamp without actually changing the next
check time. Previously, we called `SetNextCheck` with some random
timestamp just to enforce the checker to either pick it up immediately
or at a specific time. Then at some point in time, subscribing to the
`OnNextCheckChanged` signal became effectively unusable for any other
purpose than to inform the checker about a new next check time. Thus,
it resulted in introducing a new signal that is solely responsible for
informing the Icigna DB and IDO about a new next check time in places
where calling `SetNextCheck` did make sense. This commit does quite the
opposite: it replaces all calls to `SetNextCheck` that were only used to
inform the checker about a new next check time wit `OnRescheduleCheck`
calls. Only places where we actually wanted to change the next check time
still call `SetNextCheck` and thus inform the checker and all other listeners
about the new next check time. And as a bonus point, we now got rid of the
two object locks for child and parent at the same time.
This commit changes the ordering of CheckableScheduleInfo in the
multi-index container to ensure that checkables with running checks are
pushed to the end of the ordering. This prevents them from being
prioritized for scheduling ahead of others, which could lead to
unnecessary CPU load due to repeated scheduling attempts. By using a
very large value for the index of checkables with running checks, they
are effectively deprioritized until their current check is completed and
they can be reinserted with their actual next check time.
