Make Candidate an interface

[Sean-Der]

This change will allow us to have custom logic and members
per interface type. Relay candidates will have a completely different
read loop, and candidate specific state.

Relates to #47

[Sean-Der]

This diff is mostly me moving files around. I would say the big thing I would like to know is if people agree with how this is implemented.

Most candidates share all the same logic, but TURN needs to have its own readLoop and will have state not shared by other candidates.

[codecov-io]

Codecov Report

Merging #48 into master will increase coverage by 0.43%.
The diff coverage is 89.85%.

@@            Coverage Diff            @@
##           master     #48      +/-   ##
=========================================
+ Coverage   75.27%   75.7%   +0.43%     
=========================================
  Files          16      20       +4     
  Lines        1189    1202      +13     
=========================================
+ Hits          895     910      +15     
+ Misses        237     235       -2     
  Partials       57      57

Impacted Files	Coverage Δ
selection.go	75.24% <100%> (ø)	⬆️
agent.go	79.33% <100%> (-0.17%)	⬇️
candidatepair.go	86.53% <100%> (ø)	⬆️
gather.go	34.53% <50%> (-0.22%)	⬇️
candidate_host.go	84.61% <84.61%> (ø)
candidate_relay.go	88.23% <88.23%> (ø)
candidate_peer_reflexive.go	88.23% <88.23%> (ø)
candidate_server_reflexive.go	88.23% <88.23%> (ø)
candidate_base.go	89.71% <89.71%> (ø)
... and 5 more

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[hugoArregui]

I have no strong opinion about this change mainly because I don't know what's involved for the relay candidates. If the only change is the readLoop and the state maybe you could consider abstracting that out? (something like the thing we did for the agent selector)

[Sean-Der]

@hugoArregui thanks for the review (speaking of ICE reviews we need to land all your work!)

As long as you think the code is still pleasant to work with I would like to keep going with it. The relay change will involve

• Custom read behavior
• Custom write behavior
• Custom behavior when creating new candidates (for each candidate of other type we need to create a new permission on our TURN server)

[enobufs]

Later might be better than never... I support making Candidate an interface.