Truncation & EDNS0.

Counting truncation while adding RRSets into the packet.
Some RRSets are added/omitted as a whole, some per RR (should
  change later maybe).
Using the edns_udp_payload field of the packet to hold maximum
  allowed size for the packet. Overriding before converting to wire
  format.
RRSets' size is counted by iterating over RRs - this is redundant,
  we may also copy the RRs in the same time (which would however
  result in having incomplete RRSets in the response).
Setting TC bit when omitting referral NS, glue or anything from
  Answer section.