Not enough memory to parse DNS packets in resolver (thanks Frank Wiersma)

[bennylp]

This is copy of ticket #156 for trunk.

[bennylp]

Fixed in r1031:

• Optimization in DNS packet data structure to conserve memory
• Enlarge the size of temporary pool to parse message from 1500 to 4000 (this value is now also controlled by PJ_DNS_RESOLVER_TMP_BUF_SIZE configuration macro.
• Added flags in pj_dns_packet_dup() to allow exclusions of particular sections in DNS packet (for example, the query, NS, and additional info sections don't need to be kept in a cache in resolver).

Testing:

• A DNS response with 7 answers, 7 NS records, and 7 additional info records now only need ~1600 bytes of temporary memory to parse it (compared to >1800 previously)
• Only ~480 bytes of memory is needed to store DNS packet with 7 answer records (previously PJ_ENOMEM will be returned with 512 memory buffer).