Finish EDNS0 Support about dns HOT 2 OPEN

Basic EDNS0 support merged. But detection of nameservers not supporting EDNS0 is minimal, so automatic fallback is not complete. Leaving this ticket open for now. More refactoring is required in the core state machine before better detection can be cleanly added, and some of that refactoring will come with future feature merges.

from dns.

Some TODO items:

• We don't attempt to disable EDNS0 on timeouts, though some servers will fail to reply to requests with an OPT RR.
• Some servers don't echo the question section when replying with "Format error" (FORMERR). But we drop all reply packets on the ground which don't match the sent question. This results in a query timeout within the core state machine. But we don't yet disable EDNS0 on timeouts.

from dns.