concurrent.futures utilities

[jneeven]

To add:

• MetaDataFuture subclass of Future that allows passing metadata on executor.submit.
• Custom executor subclass to allow for the above, as well as:
  • Support calling unpickleable functions by passing them to the executor rather than the submit function (as with paraloop)
  • Support passing a generator to map that will be run on its own thread to allow for yielding futures even before the "future generator" is depleted and without blocking the main thread
  • Support scheduling tasks rather than submitting them right away? Can use sched and use syntax similar to schedule.