Fix per-path minimal value contribution during route finding

[tnull]

Fixes #1520

This PR ensures that we don't return an MPP route with more paths than ChannelManager would actually send payments over.

[TheBlueMatt]

SGTM, could use a test, though, IMO.

[tnull]

Since fixating the maximum number of MPP payment paths at 10 seemed at the same time rather arbitrary but also a sane default, I now opted to make max_mpp_path_count a user configurable option still defaulting to 10. This also allows me to choose different values during testing.

The minimal_value_contribution is currently however still depending on it (value/max_mpp_path_count) to ensure that we never return routes that violate the max path count limit.

[tnull]

Squashed commits.

[codecov-commenter]

Codecov Report

Merging #1526 (4617555) into main (6e00c28) will increase coverage by 0.02%.
The diff coverage is 90.62%.

❗ Current head 4617555 differs from pull request most recent head 3380687. Consider uploading reports for the commit 3380687 to get more accurate results

@@            Coverage Diff             @@
##             main    #1526      +/-   ##
==========================================
+ Coverage   90.95%   90.97%   +0.02%     
==========================================
  Files          80       80              
  Lines       43431    43661     +230     
  Branches    43431    43661     +230     
==========================================
+ Hits        39503    39722     +219     
- Misses       3928     3939      +11     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	84.34% <ø> (+0.01%)	⬆️
lightning/src/routing/router.rs	92.46% <90.62%> (-0.01%)	⬇️
lightning/src/ln/functional_tests.rs	96.91% <0.00%> (-0.24%)	⬇️
lightning/src/ln/functional_test_utils.rs	96.65% <0.00%> (+1.23%)	⬆️

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[TheBlueMatt]

LGTM. Feel free to squash.

[tnull]

Squashed.