Improve pipeline thread consumer task cancellation

[rjmholt]

Because the pipeline thread consumer is single-threaded and runs tasks sequentially, there's the real possibility that a single bad task could starve it.

For now, Ctrl+C should cancel whatever task is running. This may indeed include background tasks...

But some ideas for improving this are:

• A key-chord to cancel/unblock the pipeline thread when something other than console input is being run
• A monitor thread to supervise the pipeline thread that:
  • Logs when tasks drag on
  • Alerts users to long running tasks
  • Allows cancellation
  • Can completely reset the main runspace...