Summary:
This diff provides the basic interface definitions of persistent read
cache system
PersistentCacheOptions captures the persistent read cache options used to
configure and control the system
PersistentCacheTier provides the basic building block for constructing tiered
cache
PersistentTieredCache provides a logical abstraction of tiers of cache layered
over one another
Test Plan: Compile
Reviewers: sdong
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D57051
