Improved CompressedCache

Summary:
This is testing behavior that was reported in https://github.com/facebook/rocksdb/issues/111

No issue was found, but it still good to commit this and make CompressedCache more robust.

Test Plan: this is a plan

Reviewers: ljin, dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17625