Change subproc sigint handler to exit more cleanly

The very fast exit of os._exit can cause the stdlib multiproc glue code in the parent process, specifically the queues readers that shuttle results between processes, to deadlock.

It appears that using `sys.exit` in subprocesses instead allows those queues to close in a way that does not cause the parent process to deadlock, while still ensuring that subprocesses exit.

As a refresher: Even if the work function passed to subprocs catches KeyboardInterrupt, the multiprocessing queue code still throws, so a signal handler of some kind is needed to prevent $numproc stacktraces being printed on interrupt.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/56935938

Reviewed at https://rbcommons.com/s/twitter/r/2024/

src/python/pants/base/worker_pool.py

@@ -181,7 +181,7 @@ class SubprocPool(object):
   @staticmethod
   def worker_init():
     # Exit quietly on sigint, otherwise we get {num_procs} keyboardinterrupt stacktraces spewn
-    signal.signal(signal.SIGINT, lambda *args: os._exit(0))
+    signal.signal(signal.SIGINT, lambda *args: sys.exit())
 
   @classmethod
   def foreground(cls):