sectorstorage: Fix manager restart edge-case

[magik6k]

Fixes an issue where:

• We were running some task on the worker
• Manager would restart
• Worker would deliver the result
• Manager would start waiting for the result after it was deliver
• The task would hang ~forever

Made pretty painful by the fact that it's easy to trigger when we're restarting with a worker which is running Commit2 and has sector data on it:

• Worker is running C2
• Manager restarts
• Manager calls C1, hangs because the worker needs to finish C2
• Worker finishes C2, returns to manager
• C1 then runs on the worker, returns just fine
• C2 hangs due to this bug

[magik6k]

(verified to work on real setup with the Commit1/2 scenario)