resolved: optimize layout of DnsPacket

We obviously have lots of those, so even small savings add up.

Bitfields are dropped because they don't give any memory savings due to
alignment requirements (but would still require more complex to access).

/* size: 184, cachelines: 3, members: 28 */
/* sum members: 172, holes: 1, sum holes: 4 */
/* sum bitfield members: 4 bits (0 bytes) */
/* padding: 7 */
/* bit_padding: 4 bits */
    ↓
/* size: 176, cachelines: 3, members: 28 */