Add GracefulClose

[edaniels]

This is part of the epic to get a PeerConnection to have a GracefulClose.

[codecov]

Codecov Report

Attention: Patch coverage is 85.36585% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.36%. Comparing base (f15ba98) to head (a5212b0).

Files	Patch %	Lines
agent_handlers.go	79.31%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #712      +/-   ##
==========================================
+ Coverage   79.21%   79.36%   +0.15%     
==========================================
  Files          41       41              
  Lines        3743     3780      +37     
==========================================
+ Hits         2965     3000      +35     
- Misses        547      551       +4     
+ Partials      231      229       -2     

Flag	Coverage Δ
go	79.36% <85.36%> (+0.15%)	⬆️
wasm	22.47% <29.26%> (+0.09%)	⬆️

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