Fake consistent store

It's not a copy c'tor - it takes a T rather than an EntryHandle<T>.
It's explicit because of the style guide entry about single arg c'tors, maybe that's less important since it's private but doesn't hurt AFAIK.
It's mainly used for creating a new EntryHandle for a T which hasn't yet ever been stored (see etcd_consistent_store-inl.h#49)