ApplyToAllCacheEntries

Summary: Added a method that executes a callback on every cache entry.

Test Plan: added a unit test

Reviewers: haobo

Reviewed By: haobo

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D18441
link with lz4

Summary: link our fbcode build with lz4

Test Plan: compiles

Reviewers: sdong, haobo, dhruba, yhchiang

Reviewed By: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18447