Add notification to the middleware of peer removal.

[tmiv]

Description

This change allows data channel middleware to register a callback for when a peer disconnects.

Reference issue

Fixes #572

[codecov]

Codecov Report

Merging #573 (27905a4) into master (d7b670b) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

❗ Current head 27905a4 differs from pull request most recent head 253755f. Consider uploading reports for the commit 253755f to get more accurate results

@@            Coverage Diff             @@
##           master     #573      +/-   ##
==========================================
- Coverage   36.26%   36.15%   -0.12%     
==========================================
  Files          25       25              
  Lines        2647     2650       +3     
==========================================
- Hits          960      958       -2     
- Misses       1555     1559       +4     
- Partials      132      133       +1     

Impacted Files	Coverage Δ
pkg/sfu/datachannel.go	30.76% <0.00%> (-2.57%)	⬇️
pkg/sfu/subscriber.go	58.22% <0.00%> (-0.75%)	⬇️
pkg/sfu/router.go	46.82% <0.00%> (-1.16%)	⬇️

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[OrlandoCo]

Hey @tmiv sorry for late response, but is necessary to put the callback on the datachannel?, should Peer be a better location for a close callback?

[tmiv]

No @OrlandoCo, I don't think so. It's the middleware who should process the disconnect (onRemovePeer). Just as the middleware processes the messages (onMessage). But I think that I might be convoluting the concept of a dc middleware here. It seems that data channel can have more than one middleware thus the DC chain structure. Also, that the concept of a middleware is really just a single function.