StoreGate: Some adjustments for Handle ctors.

A Handles can be initialized from a HandleKey; in that case, it retains a reference to the HandleKey. But that's problematic if the HandleKey was a temporary. Add deleted ctors taking rvalue references to disallow passing a temporary for a HandleKey.

Read/Write/UpdateHandle can in addition be directly initialized from strings. For consistency, add versions of these ctors that also take and EventContext.