Improve performance of UDPMux map lookups

[potaga]

Description

UDPMux is using a map to lookup addresses of each packets. Unfortunately the key is based on a string and each time we want to check the map, a conversion of the UDP address to string is made (.String()) which is expensive.
This CR replace the string key by a binary key called ipPort. This structure contains a netip.Addr field and ipPort could be used as a map key

[codecov]

Codecov Report

Attention: Patch coverage is 57.50000% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 78.42%. Comparing base (52f2075) to head (430a9a0).

Files	Patch %	Lines
udp_mux.go	62.50%	5 Missing and 4 partials ⚠️
udp_muxed_conn.go	50.00%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   78.69%   78.42%   -0.27%     
==========================================
  Files          42       42              
  Lines        4464     4487      +23     
==========================================
+ Hits         3513     3519       +6     
- Misses        733      745      +12     
- Partials      218      223       +5     

Flag	Coverage Δ
go	78.42% <57.50%> (-0.27%)	⬇️
wasm	23.71% <0.00%> (-0.13%)	⬇️

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