Limit maximum CLTV delta during routing.

[tnull]

This PR attempts to address #996. As this is my first contact with the ~950 lines of get_route, I hope that I didn't mess up too much. I also fixed some typos I found while going through the code.

Three remarks though:

1. Currently a lot of unit tests fail. This seems to be the case since many of the supplied cltv_expiry_delta values in the dummy graph data are chosen very high. If all agree that the failures do not come from something else, I could change them to some sane(r)(?) values, even though I'm not entirely sure yet what they would be.
2. How should we choose the MAX_TOTAL_CLTV_DELTA limit? The value of 1000 seems like a good starting point, however, a decent number of nodes use values that are close enough or higher than that (see attached data I took from a recent graph snapshot). These might be excluded from any payment paths, which could impair connectivity and payment success.
3. In order to be minimal invasive I currently set this MAX_TOTAL_CLTV_DELTA limit inline. But maybe this parameter should be configurable, i.e., by defining it as part of RouteParameters or a new RoutingConfig in config.rs?

[TheBlueMatt]

This looks great! Some minor comments and could use a test to demonstrate it works (plus fix the tests that are failing), but looks great.

[TheBlueMatt]

Basically looking good, needs to be configurable and needs a test for the max-expiry behavior specifically.

[codecov-commenter]

Codecov Report

Merging #1234 (f4e57f9) into main (10204f7) will decrease coverage by 0.00%.
The diff coverage is 62.06%.

❗ Current head f4e57f9 differs from pull request most recent head 367ed31. Consider uploading reports for the commit 367ed31 to get more accurate results

@@            Coverage Diff             @@
##             main    #1234      +/-   ##
==========================================
- Coverage   90.41%   90.41%   -0.01%     
==========================================
  Files          70       70              
  Lines       38087    38116      +29     
==========================================
+ Hits        34437    34463      +26     
- Misses       3650     3653       +3     

Impacted Files	Coverage Δ
lightning/src/routing/router.rs	91.89% <62.06%> (-0.17%)	⬇️
lightning/src/ln/channel.rs	89.35% <0.00%> (-0.03%)	⬇️
lightning/src/ln/functional_tests.rs	97.36% <0.00%> (+0.06%)	⬆️

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[tnull]

Basically looking good, needs to be configurable and needs a test for the max-expiry behavior specifically.

I now added a simple test against the default value and limit of 0 with 0c93422.
Let me know if you think more in-depth testing is desirable.

[jkczyz]

Looks pretty good so far. Thanks for putting this together!

[TheBlueMatt]

LGTM. A few super-trivial comments that might as well be addressed when squashing, but I'd let @jkczyz follow up first.

[tnull]

Rebased and squashed now, should be ready to merge.

[TheBlueMatt]

One API comment, otherwise ACK

[jkczyz]

Yeah, looks good module some nits.

[jkczyz]

ACK 367ed31