Implemented unicast responses to direct unicast, unicast questions and legacy unicast queries.

[pixsperdavid]

Draft for now as I now need to construct some tests for this to verify it functions as expected.

One missing area for now is the following from RFC 6762:

When receiving a question with the unicast-response bit set, a
responder SHOULD usually respond with a unicast packet directed back
to the querier. However, if the responder has not multicast that
record recently (within one quarter of its TTL), then the responder
SHOULD instead multicast the response so as to keep all the peer
caches up to date, and to permit passive conflict detection. In the
case of answering a probe question (Section 8.1) with the unicast-
response bit set, the responder should always generate the requested
unicast response, but it may also send a multicast announcement if
the time since the last multicast announcement of that record is more
than a quarter of its TTL.

Implementing the above will require storing the time in which each service record was last multicasted on each specific interface so is therefore quite complex. For now I have simply used the logic that if all the questions in a given query request a unicast response, a unicast response is sent.

[keepsimple1]

Thank you for the PR! While you're working on adding the tests, here are a few comments inline.

[keepsimple1]

Implementing the above will require storing the time in which each service record was last multicasted on each specific interface so is therefore quite complex. For now I have simply used the logic that if all the questions in a given query request a unicast response, a unicast response is sent.

Sounds good to me, to implement this part later / separately.

[keepsimple1]

Thanks for making the changes. Looks good!