Add Memory Isolating Cache Proxy

Created a specialized dogpile.cache proxy to isolate in-memory data in the
cache region from modificaiton during tests using liberal application of
copy.deepcopy for get/set operations.

partial-blueprint: caching-layer-for-driver-calls
Change-Id: Id6d27219b75ea7e6709d220542dd31c4a6c7aa04
Merge "Add Memory Isolating Cache Proxy"