Make (statically) return type of VBucket::addTempStoredValue to just NOMEM or BG_FETCH

enum class AddStatus : uint16_t {

    Success, //!< Add was successful.

    NoMem, //!< No memory for operation

    Exists, //!< Did not update -- item exists with this key

    UnDel, //!< Undeletes an existing dirty item

    AddTmpAndBgFetch, //!< Create a tmp item and schedule a bg metadata fetch

    BgFetch //!< Schedule a bg metadata fetch to process ADD op

};