agent: better handling of deletions

[psFried]

There's a few different race conditions around deletions of live specs:

• Dangling internal.tasks rows - these happen when something calls send_message_to_task while the controller is running the final cleanup/deletion of a live spec.
• Dangling journals - these happen when the collection controller deletes the collection journals prior to the capture fully stopping.

Neither of these cause any major issues with tasks, but they are noisy and create false positive alerts.