Allow to open multiple connections via NAT hole punching

[soffokl]

Closes #1834

[codecov-io]

Codecov Report

Merging #1887 into master will increase coverage by 0.05%.
The diff coverage is 66.03%.

@@            Coverage Diff             @@
##           master    #1887      +/-   ##
==========================================
+ Coverage   47.17%   47.23%   +0.05%     
==========================================
  Files         288      288              
  Lines       11775    11753      -22     
==========================================
- Hits         5555     5551       -4     
+ Misses       5787     5771      -16     
+ Partials      433      431       -2     

Impacted Files	Coverage Δ
nat/traversal/noop.go	0.00% <0.00%> (ø)
services/wireguard/service/service_unix.go	16.66% <0.00%> (+0.74%)	⬆️
nat/traversal/pinger.go	75.73% <78.04%> (+2.40%)	⬆️
consumer/bandwidth/tracker.go	100.00% <100.00%> (ø)
session/manager.go	71.87% <100.00%> (ø)
core/discovery/discovery.go	66.38% <0.00%> (+0.84%)	⬆️
core/quality/mysterium_morqa.go	68.85% <0.00%> (+1.63%)	⬆️

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