Aborting

I am keeping track of how abort should be implemented here (these sat on my whiteboard for too long and are taking precious space):

• Make sure that filters clean their output if aborted, maybe executive should do this
• Pipeline should not propagate (i.e. pipeline execution should stop once aborted, not just the current filter)
• What about mtime? This has to be handled properly so that the aborted part of the pipeline re-executes properly later
• How is update suppressor involved in all of this?
• How do we notify the server?