Find path will try other channels considering busyness level

[chenyukang]

Find path will track how many payments are pending on a channel, then try to pick other channel which is not busy.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.82%. Comparing base (fad9612) to head (8cf88c4).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #569      +/-   ##
===========================================
+ Coverage    51.75%   51.82%   +0.07%     
===========================================
  Files           52       52              
  Lines        34277    34305      +28     
===========================================
+ Hits         17739    17778      +39     
+ Misses       16538    16527      -11     

Flag	Coverage Δ
unittests	51.82% <100.00%> (+0.07%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[chenyukang]

I refactor testing code to add benchmark testing, from the result, we can get:

1. the performance for multiple channel scenarios improved
2. the performance does not change when channel number is 1
3. performance will be improved much more when the number of channels increasing

[doitian]

Could you please explain the algorithm to find the route based on busyness level?

[chenyukang]

https://github.com/nervosnetwork/fiber/pull/569/files#diff-ac11142a8dee80631cd1352a43e26ba373f19dbd6009bcc71c66812b508ca47cR795-R811

Supposing there are multiple channels between two nodes, use these two functions to track the inflight payment number of a channel(an inflight payment means payment is sent out but haven't received a result).

if we have the inflight number for a channel, we can add a penalty when calculating the weight for a channel in path finding:
https://github.com/nervosnetwork/fiber/pull/569/files#diff-ac11142a8dee80631cd1352a43e26ba373f19dbd6009bcc71c66812b508ca47cR1354-R1370