Added debug code exhibiting perfmon_collection_t lifetime bug.
Moved perfmon_collection and perfmon_membership (and cache_order_source, why not) before the serializer and cache fields in internal_disk_backed_queue_t.
Uncommented the rassert(empty()) line in ~intrusive_list_t.
Merge branch 'next' into sam_1497
Removed perfmon_collection_t::is_not_destroyed field (which is redundant with intrusive_list_t non-empty checks, for our purposes).
Merge branch 'sam_1497' into next