Use case:
Destructive tests

Behavior:
- When set, the TaskToRun for the corresponding TaskSlice can only reaped by a bot managed by MP.
- When set, immediately after the task is completed, the bot is deleted by Swarming itself. The deletion causes MP to reap it and the VM is recycled.

Advantages:
- Simpler to implement than  issue 669660 .
- Does not enforce that the bot itself is single use, making better use of the whole fleet; a bot could run a few normal task, then a destructive test.
- The reverse can be true, a subset of the fleet can be assigned to destructive tasks, but that's not an hard requirement by design.
- Does not require MP changes, it's a pure Swarming only implementation that is easy to understand.
- The task may or may not also be marked as idempotent. That's up to the user.