Summary:
We expect the persistent read cache to perform at speeds upto 8 GB/s. In order
to accomplish that, we need build a index mechanism which operate in the order
of multiple millions per sec rate.

This patch provide the basic data structure to accomplish that:

(1) Hash table implementation with lock contention spread
    It is based on the StripedHashSet<T> implementation in
    The Art of multiprocessor programming by Maurice Henry & Nir Shavit
(2) LRU implementation
    Place holder algorithm for further optimizing
(3) Evictable Hash Table implementation
    Building block for building index data structure that evicts data like files
    etc

TODO:
(1) Figure if the sharded hash table and LRU can be used instead
(2) Figure if we need to support configurable eviction algorithm for
EvictableHashTable

Test Plan: Run unit tests

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D55785