Various crash consistency bugs for pslab memcached.

[iangneal]

Bug 1

• CAS ID is not persisted, potentially causing atomicity problems for client operations if the persistent memcached service experiences a failure and restarts.
  

  memcached-pmem/items.c

  Lines 537 to 538 in 8f121f6

  	/* Allocate a new CAS ID on link. */
  	ITEM_set_cas(it, (settings.use_cas) ? get_cas_id() : 0);

Bug 2

• CAS ID is updated after item is linked, so even if Bug 1 is fixed (i.e., CAS ID field is made persistent), then it is still possible for the item to be linked before the CAS ID is persisted, leading to the same issue as Bug 1.
  

  memcached-pmem/items.c

  Lines 537 to 538 in 8f121f6

  	/* Allocate a new CAS ID on link. */
  	ITEM_set_cas(it, (settings.use_cas) ? get_cas_id() : 0);

Bug 3

• Time field is updated after item is linked. On a crash, this could cause the item to have an uninitialized time, causing it to be evicted from the service earlier than expected.

memcached-pmem/items.c

Lines 519 to 522 in 8f121f6

	it->it_flags |= ITEM_LINKED;
	pmem_member_persist(it, it_flags);
	it->time = current_time;
	pmem_member_persist(it, time);

memcached-pmem/items.c

Lines 134 to 136 in 8f121f6

	if ((it->time <= oldest_live)
	|| (oldest_cas != 0 && cas != 0 && cas < oldest_cas)) {
	return 1;