Summary:
This provides provides an implementation of PersistentCacheTier that is
specialized for RAM. This tier does not persist data though.

Why do we need this tier ?

This is ideal as tier 0. This tier can host data that is too hot.

Why can't we use Cache variants ?

Yes you can use them instead. This tier can potentially outperform BlockCache
in RAW mode by virtue of compression and compressed cache in block cache doesn't
seem very popular. Potentially this tier can be modified to under stand the
disadvantage of the tier below and retain data that the tier below is bad at
handling (for example index and bloom data that is huge in size)

Test Plan: Run unit tests added

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D57069