Send invoice if threshold reached

[vkuznecovas]

Invoices will still be sent periodicly, but if a spike occurs an invoice now might be sent earlier.

Fixes #2255

[codecov-commenter]

Codecov Report

Merging #2270 into master will increase coverage by 0.22%.
The diff coverage is 68.96%.

@@            Coverage Diff             @@
##           master    #2270      +/-   ##
==========================================
+ Coverage   47.73%   47.96%   +0.22%     
==========================================
  Files         273      273              
  Lines       12715    12775      +60     
==========================================
+ Hits         6070     6127      +57     
- Misses       6052     6058       +6     
+ Partials      593      590       -3     

Impacted Files	Coverage Δ
config/flags_payments.go	0.00% <0.00%> (ø)
session/pingpong/factory.go	0.00% <0.00%> (ø)
session/pingpong/invoice_tracker.go	68.07% <72.72%> (+2.28%)	⬆️
mobile/mysterium/wireguard_connection_setup.go	29.45% <0.00%> (-0.62%)	⬇️
services/wireguard/service/service_unix.go	21.91% <0.00%> (-0.16%)	⬇️
config/flags_service_wireguard.go	0.00% <0.00%> (ø)
services/wireguard/wireguard.go	87.09% <0.00%> (+0.57%)	⬆️
p2p/channel.go	75.09% <0.00%> (+0.76%)	⬆️
core/discovery/discovery.go	65.57% <0.00%> (+0.81%)	⬆️
services/wireguard/service/options.go	51.85% <0.00%> (+1.85%)	⬆️
... and 1 more

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