Fix and test in-flight HTLC scoring in between retries

[valentinewallace]

Turns out that untested code really is broken code.

I think we don't need to do something similar for the first_hops because retries know what scids failed previously. That doesn't cover all cases, but on the other hand it saves a vec allocation to not recompute first hops and should be rare, so I think it's good enough(?).

Partially addresses #1932.

[codecov-commenter]

Codecov Report

Base: 87.33% // Head: 87.31% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (3ddb8ff) compared to base (be4bb58).
Patch coverage: 87.87% of modified lines in pull request are covered.

❗ Current head 3ddb8ff differs from pull request most recent head 2b2965f. Consider uploading reports for the commit 2b2965f to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2020      +/-   ##
==========================================
- Coverage   87.33%   87.31%   -0.03%     
==========================================
  Files         100      100              
  Lines       44160    44168       +8     
  Branches    44160    44168       +8     
==========================================
- Hits        38568    38566       -2     
- Misses       5592     5602      +10     

Impacted Files	Coverage Δ
lightning/src/ln/functional_tests.rs	97.20% <ø> (-0.01%)	⬇️
lightning/src/ln/onion_route_tests.rs	95.25% <ø> (ø)
lightning/src/ln/shutdown_tests.rs	96.36% <ø> (-0.01%)	⬇️
lightning/src/util/test_utils.rs	67.02% <82.50%> (+2.07%)	⬆️
lightning/src/ln/outbound_payment.rs	80.28% <92.30%> (+0.08%)	⬆️
lightning-invoice/src/utils.rs	96.14% <100.00%> (ø)
lightning/src/ln/channelmanager.rs	86.91% <100.00%> (-0.02%)	⬇️
lightning/src/ln/functional_test_utils.rs	88.65% <100.00%> (+0.17%)	⬆️
lightning/src/ln/payment_tests.rs	96.59% <100.00%> (+0.01%)	⬆️
lightning/src/routing/router.rs	92.65% <100.00%> (-0.16%)	⬇️
... and 4 more

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[TheBlueMatt]

Ah the ancient proverbs strike agian.

Needs rebase already lol. Basically LGTM tho, but no-std is sad.

[valentinewallace]

Tacked on a small commit to remove unnecessary (as of #1996) Router trait methods

[TheBlueMatt]

Needs rebase :(

[valentinewallace]

Rebased

[TheBlueMatt]

Note that this only actually tests the InFlightHtlcs creation in send_payment_with_retry. It would be nice to test the two other cases where we have a similar constructor in channelmanager. Can happen later though.

[TheBlueMatt]

CI failure is #2032, will need to kick it after other jobs complete, nbd.