Possible bug in surveyor virtual machine cleanup task scheduling

Spurious results indicate the VM-backed surveyor misses occasional tasks during cleanup. To diagnose further, try the following:

<pre>
python -m pomagma init skrj  # or another theory
python -m pomagma update-theory skrj dry_run=True
</pre>

If all is working correctly, `update-theory` prints 'theory did not change'.
