manager: prevent cleanup of triggering units before we start the handler

manager: prevent cleanup of triggering units before we start the handler

This fixes the following case:
OnFailure= would be spawned correctly, but OnSuccess= would be
spawned without the MONITOR_* metadata, because we'd "collect" the unit
that started successfully. So let's block cleanup while we have a job
running for the handler. The job cannot last infinitely, so at some point
we'll be able to collect both.