Improve the handling of sync child jobs to ensure it behaves correctly
in all scenarios.
The sync service now uses a isolated context to avoid all the pipeline
stages shutting down before all the sync tasks have had the opportunity
to run their course.
The job waiter now immediately starts with a counter of 1 and waits
until all the child and the parent job finish before considering the
work to be finished.
Finally, we also handle the case where a sync job can't be queued
because the calling context has been cancelled.
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