Exit workers on sigint rather than ignore

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/42139656

Also: ctrl-c'ed in the middle of cache fetch with and without this change.

Without: observe dots continue to print for several seconds after main process appears to exit. `killall python` required to stop them.
With: dots stop printing immediately, no python processes running.

Bugs closed: 829

Reviewed at https://rbcommons.com/s/twitter/r/1405/
Sync of https://github.com/pantsbuild/pants/commit/064b2212376ea8d522dcfc796a233861ebff46a0