Support dig based unicast requests

[edaniels]

e.g. dig -p 5353 @224.0.0.251 pion-test.local

Description

I noticed this wasn't supported after getting IPv6/unicast support in. The basic issue was 1. not using the ID that the question came from in the answer and 2. not supporting legacy resolvers (technically like ourselves as a non-continuous querier)

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (23c33ce) 67.50% compared to head (70b6f53) 66.75%.

Files	Patch %	Lines
conn.go	53.57%	8 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
- Coverage   67.50%   66.75%   -0.75%     
==========================================
  Files           1        1              
  Lines         717      737      +20     
==========================================
+ Hits          484      492       +8     
- Misses        165      173       +8     
- Partials       68       72       +4     

Flag	Coverage Δ
go	66.75% <53.57%> (-0.75%)	⬇️

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