KeyBuilder and hashed keys

[JunTaoLuo]

Currently keys are strings. We may want to consider creating an abstraction of KeyBuilder that only creates a string for IDistributedCache based stores and use an object as the key for IMemoryCache based stores. We may also want to consider adding support for hashing the keys to a constant length

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2616