Cleanly close agent goroutines

[edaniels]

This ensures no goroutines are running when an ICE agent closes. I'd also like to backport this into v2 so that webrtc/v3 can get it.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.07%. Comparing base (2a9fdb5) to head (72c7732).

Files	Patch %	Lines
agent_handlers.go	67.85%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
- Coverage   79.19%   79.07%   -0.12%     
==========================================
  Files          41       41              
  Lines        3739     3771      +32     
==========================================
+ Hits         2961     2982      +21     
- Misses        547      557      +10     
- Partials      231      232       +1     

Flag	Coverage Δ
go	79.07% <75.00%> (-0.12%)	⬇️
wasm	22.23% <30.55%> (+0.10%)	⬆️

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[davidzhao]

@paulwe tagging you on this as it closely matches with your initial comments about not leaving behind rogue goroutines