resolved: optimize layout of DnsTransaction

/* size: 296, cachelines: 5, members: 46 */
/* sum members: 278, holes: 5, sum holes: 17 */
/* sum bitfield members: 4 bits, bit holes: 1, sum bit holes: 4 bits */
    ↓
/* size: 280, cachelines: 5, members: 46 */
/* sum members: 278, holes: 1, sum holes: 1 */
/* sum bitfield members: 4 bits, bit holes: 1, sum bit holes: 4 bits */