Alloc_pool_item_t sequence number should be atomic

Definition
  Line 55: uintmax_t sequence;  // uintmax_t ensures proper alignment of following data
In qd_alloc:
  Line 406: item->sequence = 0;
in qd_dealloc:
  Line 477: item->sequence++;
In qd_alloc_sequence:
  Line 525: reuturn item->sequence;