Add support for SetIfNotExists

[rlinehan]

SetIfNotExists is a new API that stores a value for a key if the key is not already present (and returns Stored along with the key and value), otherwise it does not store the value (and returns NotStored).

Although it's not really to do with "data structures," this API is only available in caches that also support data structures.

[nand4011]

Closing in favor of a general ticket to implement the missing scalar methods: #164