ASan: object type mismatch in ConnHandler::addStats when called via KVBucket::snapshotStats

As part of http://review.couchbase.org/95248 a ConnHandler's priority was included in the connection stats; this priority was fetched from the underlying Connection via it's cookie - c below:

void ConnHandler::addStats(ADD_STAT add_stat, const void* c) {

    ...

    const auto priority = engine_.getDCPPriority(c);

    const char* priString = "<INVALID>";

    switch (priority) {

        ... Map priority to a string ...

    }

    addStat("priority", priString, add_stat, c);

}

However this is the wrong trousers^Wcookie; the cookie passed to ADD_STAT is not necessarily a memcached connection cookie; indeed in the case of KVBucket::snapshotStats() is is an instance of snapshot_stats_t.

As a result, this triggers a crash under AddressSanitizer as we treat a snapshot_stats_t object as a memcached connection cookie and call cookie_get_priority on it - this in turn attempts to dereference as a Cookie object and reads garbage data.