Make file caches more flexible
Summary:
- Allow caller to set an acceptable lock wait time.
- If we fail to lock and don't need to hold a lock for a write, just miss the cache.
- See discussion in D5644.
Test Plan: I think I ran it? I'll actually check this in the next diff.
Reviewers: vrana
Reviewed By: vrana
CC: aran
Differential Revision: https://secure.phabricator.com/D5657