This issue replaces the juicy
HBASE-7233 which started undoing KeyValue as a Writable and ended up as a general discussion of KeyValue serialization. Along the way it served as a scratch sheet for how to do lots of KeyValues across RPC. This issue is the umbrella issue for making it so we can pass Cells over our protobuf'd RPC rather than KeyValues; i.e. an Interface which could be differently implemented rather than our current non-evolvable KeyValue serialization.