Known-Answer suppression

[quilir]

Currently, the crate does not implement the important feature of the mDNS - https://tools.ietf.org/html/rfc6762#section-7.1

It reduces network usage and is necessary element of mDNS. It is marked as MUST.

Responder should check answer section of the query. If he has requested service, he:

• MUST NOT include it when the answer service RR TTL is < half TTL
• MUST include it otherwise (not in answer section or RR TTL >)