Summary:
1. verify that we cache entries on background.
2. verify that we don't fill the cache when we can't take the lock.
Test Plan: passed existing unit tests.
Reviewers: durham, lcharignon
Reviewed By: lcharignon
Subscribers: mitrandir, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D3471355
Signature: t1:3471355:1466719450:191070cd2d5ef93e0e1a6925a04d1a885431cbb4
