Implement RTT calculation for candidate pair stats

[kcaffrey]

Implement CurrentRoundTripTime, TotalRoundTripTime, RequestsSent, and ResponsesReceived for candidate pair statistics.

This change on its own is not quite enough to get stats working end to end. Notably, (*Agent).GetSelectedCandidatePair() copies both candidates, which due to internally marshaling and unmarshaling the candidate to implement the copy, ends up generating new candidate ids.

Because pion/webrtc stores stats in a map based on candidate ids, this means that it is currently impractical to actually access these stats.

[codecov[bot]]

Codecov Report

Merging #420 (1c3436d) into master (4f34006) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   79.31%   79.33%   +0.02%     
==========================================
  Files          33       33              
  Lines        3650     3659       +9     
==========================================
+ Hits         2895     2903       +8     
- Misses        589      590       +1     
  Partials      166      166              

Flag	Coverage Δ
go	79.33% <100.00%> (+0.02%)	:arrow_up:
wasm	26.70% <15.15%> (+0.07%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
agent.go	81.25% <100.00%> (+0.40%)	:arrow_up:
agent_stats.go	84.37% <100.00%> (ø)
candidatepair.go	84.48% <100.00%> (+1.46%)	:arrow_up:
selection.go	80.37% <100.00%> (-1.71%)	:arrow_down:

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[jech]

Hmm... you're using just the last sample, which might yield an extremely noisy signal. Shouldn't we be computing an exponential average instead? (Please see Figure 6 of https://arxiv.org/pdf/1403.3488.pdf for an illustration of what I mean.)