Comment 8 for bug 1211276

Steve, here's my tentative plan:

- API sends action to engine (round-robin)
- Engine checks status of stack
- IF stack status == "INPROGRESS":
  + Broadcast "Stop working on stack X" message to all engines
  + Wait for response
  + IF no response:
    * Force update stack status to "ORPHANED"
- Proceed with operation

I expect most of the work to be implementing a new "STOP" action and a new "ORPHANED" status.

Thoughts?