Use balance estimates from past payments in path-finding

[thomash-acinq]

We estimate the probability that a given route can relay a given payment as part of route selection.
Until now this estimate was naively assuming the channel balances to be uniformly distributed.
We will now use data from past payment attempts (both successes and failures) to provide a better estimate, hopefully improving route selection.

[codecov-commenter]

Codecov Report

Merging #2308 (a834222) into master (3547f87) will decrease coverage by 0.05%.
The diff coverage is 90.47%.

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@@            Coverage Diff             @@
##           master    #2308      +/-   ##
==========================================
- Coverage   85.87%   85.83%   -0.05%     
==========================================
  Files         215      215              
  Lines       17823    17826       +3     
  Branches      761      759       -2     
==========================================
- Hits        15306    15301       -5     
- Misses       2517     2525       +8     

Files Changed	Coverage Δ
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	92.99% <0.00%> (-0.30%)	⬇️
.../src/main/scala/fr/acinq/eclair/router/Graph.scala	97.00% <93.33%> (-0.34%)	⬇️
...cinq/eclair/remote/EclairInternalsSerializer.scala	97.82% <100.00%> (ø)
...scala/fr/acinq/eclair/router/BalanceEstimate.scala	98.91% <100.00%> (-0.03%)	⬇️
...cala/fr/acinq/eclair/router/RouteCalculation.scala	94.56% <100.00%> (ø)

... and 5 files with indirect coverage changes