on Linux, only process cookies if they arrived via inotify

Summary:
This is explained in the doc.

This is still broken on other platforms, but this diff doesn't make things worse.

Test Plan: `arc unit --everything` + hg test suite with `-j16`.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D923205

--HG--
extra : rebase_source : ecc207668a8e1f1ac8255629dc0b25f4eb5d733e
extra : amend_source : 9c2279a8ab4ad3f754dfdf321aa605d61a4f21e1