Provide SCTP Association OnClose callback

[sukunrt]

The only way to detect a remote connection close is to check the state of the SCTP transport.
See:
w3c/webrtc-pc#1799 (comment)

In case it wasn't clear, I'm arguing for an exception for data
channels, saying pc1.close() should fire close on both ends of a data channel, because A) that is useful, and the only way to detect remote closing of a peer connection, and B) it's a workaround for the fact that closing of peer connections isn't advertised to the other end.

Depending on datachannel close to notify us requires out of band protocol agreement as datachannels can be closed by the remote for other reasons. Having access to the SCTP layer we can provide an accurate SCTP Assocation closed event.

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (c4d56d4) to head (7b15659).

Files	Patch %	Lines
sctptransport.go	68.75%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2858      +/-   ##
==========================================
+ Coverage   78.83%   78.97%   +0.14%     
==========================================
  Files          89       89              
  Lines        8432     8447      +15     
==========================================
+ Hits         6647     6671      +24     
+ Misses       1298     1292       -6     
+ Partials      487      484       -3     

Flag	Coverage Δ
go	80.54% <68.75%> (+0.15%)	⬆️
wasm	65.11% <ø> (ø)

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[Sean-Der]

LGTM! Merge w/e CI passes

thanks for doing this @sukunrt

[sukunrt]

@Sean-Der Can we merge this? The ci issues are with err shadowing which in this case is more idiomatic.

[sukunrt]

I removed the shadowing.