Makes UDPMux IPv4/IPv6 aware

[Antonito]

UDPMux before only worked with UDP4 traffic.
UDP6 traffic would simply be ignored.

This commit implements 2 connections per ufrag. When requesting a
connection for a ufrag the user must specify if they want IPv4 or IPv6.

Relates to #518

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This is basically a port of #424 to UDPMux

I tested it on an IPv6-only network, on which I was previously unable to connect to a SFU. It seems to work great so far.

[codecov]

Codecov Report

Merging #431 (fafa43e) into master (427ac0f) will decrease coverage by 0.20%.
The diff coverage is 67.27%.

@@            Coverage Diff             @@
##           master     pion/webrtc#431      +/-   ##
==========================================
- Coverage   78.92%   78.71%   -0.21%     
==========================================
  Files          34       34              
  Lines        3886     3914      +28     
==========================================
+ Hits         3067     3081      +14     
- Misses        634      642       +8     
- Partials      185      191       +6     

Flag	Coverage Δ
go	78.71% <67.27%> (-0.21%)	⬇️
wasm	24.96% <0.00%> (-0.18%)	⬇️

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

Impacted Files	Coverage Δ
udp_mux_universal.go	75.16% <0.00%> (ø)
udp_mux.go	77.00% <67.34%> (-6.73%)	⬇️
gather.go	67.33% <100.00%> (+0.40%)	⬆️
agent.go	81.93% <0.00%> (+0.23%)	⬆️

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[Sean-Der]

@braginini I just added you to the Pion org. Would love your opinion on this! Since you have been improving this area as well.

[Sean-Der]

@Antonito woah this is fantastic! Awesome work, really exciting that we got this fixed so quickly :)

This is an approve from me. Worth waiting for @davidzhao @braginini, but if they are busy my vote is API breakage + merge!

[Antonito]

@Antonito woah this is fantastic! Awesome work, really exciting that we got this fixed so quickly :)

This is an approve from me. Worth waiting for @davidzhao @braginini, but if they are busy my vote is API breakage + merge!

Thanks a lot for the quick feedback! Really excited to get this landed, it's been an issue for a while on my SFU – I wasn't sure how to fix it, until I read your TCP PR 😁

I'll give it a couple hours and will break the API+merge if no new feedback :)

[Sean-Der]

With @braginini approval I am all for it! Feel free to tag new ice/webrtc versions as well @Antonito !

[davidzhao]

Excellent work @Antonito ! thank you for fixing this. @jech will be happy :)

[braginini]

Great work @Antonito
This will actually help us at wiretrustee once we start working on ipv6 support :)

[jech]

@jech will be happy :)

He will :-)