Fix computation of path weight

[thomash-acinq]

There are two cases depending on whether you use weight ratios or not. They used to behave very differently.

• Without weight ratios, the weight would be the total amount to send (base amount + fees)
• With weight ratios, the ratios would apply to the total amount, not just the fees. The effect is that only the number of hops and then the weight factors matter as the fee itself is negligible compared to the total amount.

The code is now shared and the weight is now the sum of the fees (multiplied by a factor if using weight ratios).

[codecov-commenter]

Codecov Report

Merging #1794 (86cb2fe) into master (55b50ec) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1794   +/-   ##
=======================================
  Coverage   89.35%   89.36%           
=======================================
  Files         143      143           
  Lines       10933    10932    -1     
  Branches      476      481    +5     
=======================================
  Hits         9769     9769           
+ Misses       1164     1163    -1     

Impacted Files	Coverage Δ
.../src/main/scala/fr/acinq/eclair/router/Graph.scala	98.14% <100.00%> (-0.02%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.39% <0.00%> (ø)
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	85.10% <0.00%> (+0.42%)	⬆️

[t-bast]

LGTM, good catch!