Restore the FSEvents-based file watcher, and remove the WinFileWatcher.

This is a cherry-pick from the 1.9.6 branch:
   3cf5710e777550cbf1b684d580da0a5f65f27b6c
   3fd0b77bed73aa7036c70bf0c4e3cfff4fd170f0
with the following modification for 1.9.10:

Instead of adding a new watcher_common.ignore_dir() method, I use the
existing (in 1.9.10) skip_ignored_dirs() method.

Auditors: dylan

Test plan:
Fingers crossed.