Summary:
We have encountered a kernel issue where `rmdir` a non-empty directory may race
with other things and hang in kernel for a long time.
This patch changes `os.rmdir` to avoid `rmdir` non-empty directories. It is
written in Cython calling the low-level `readdir` libc friends to make overhead
minimal.
Test Plan: Added a new test
Reviewers: #sourcecontrol, clm, rmcelroy
Reviewed By: rmcelroy
Subscribers: rmcelroy, simpkins, osandov, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4716711
Tasks: 16647532
Signature: t1:4716711:1489627923:7c7432748c1fd8c070ce257bd172feebd3807f65
