Revert "Re-Revert "DirectoryWatcher: add an implementation for Windows""

This reverts commit fb32de9e.

Remove the secondary synchronization point as noted by Adrian.  This is
technically only to make the builders happier about tests and should not
be needed.  This also pushes the condition variable setting to after the
watch is actually established (which was the source of the original race
condition, but would normally succeed as the thread shouldn't get put to
sleep immediately on the trigger of the condition variable).

This also was pretested on the chromium builders:
https://ci.chromium.org/ui/p/chromium/builders/try/win_upload_clang/1612/overview.