Consider maximum path length during path finding.

[tnull]

With this PR, we only consider returning paths from get_route which are shorter than our maximum path length estimate.

Rationale as given in the comment:

	// In the legacy onion format, the maximum number of hops used to be a fixed value of 20.
	// However, in the TLV onion format, there is no fixed maximum length, but the `hop_payloads`
	// field is always 1300 bytes. As the `tlv_payload` for each hop may vary in length, we have to
	// estimate how many hops the route may have so that it actually fits the `hop_payloads` field.

Fixes #1371.

[codecov-commenter]

Codecov Report

Merging #1476 (87c9684) into main (342698f) will increase coverage by 0.14%.
The diff coverage is 89.53%.

@@            Coverage Diff             @@
##             main    #1476      +/-   ##
==========================================
+ Coverage   90.89%   91.03%   +0.14%     
==========================================
  Files          76       77       +1     
  Lines       42075    43973    +1898     
  Branches    42075    43973    +1898     
==========================================
+ Hits        38244    40032    +1788     
- Misses       3831     3941     +110     

Impacted Files	Coverage Δ
lightning/src/routing/router.rs	92.52% <89.53%> (+<0.01%)	⬆️
lightning-net-tokio/src/lib.rs	75.91% <0.00%> (-0.31%)	⬇️
lightning/src/ln/functional_tests.rs	97.16% <0.00%> (-0.02%)	⬇️
lightning/src/util/time.rs	81.25% <0.00%> (ø)
lightning/src/routing/scoring.rs	96.12% <0.00%> (+1.47%)	⬆️
lightning-invoice/src/lib.rs	89.12% <0.00%> (+1.73%)	⬆️
lightning/src/ln/peer_channel_encryptor.rs	96.97% <0.00%> (+1.98%)	⬆️
lightning-invoice/src/payment.rs	95.77% <0.00%> (+3.02%)	⬆️
lightning/src/ln/peer_handler.rs	62.29% <0.00%> (+5.46%)	⬆️

---

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[tnull]

Squashed and added a minor refactoring commit, because I found spelling out these types could improve readability.

[TheBlueMatt]

LGTM, aside form some formatting nits.

[tnull]

Rebased and squashed.

[tnull]

Kicked CI...

[tnull]

Squashed & rebased.

[TheBlueMatt]

LGTM, didn't review the math too closely but its close enough, lets set the max to 19.

[tnull]

Squashed once more.

[valentinewallace]

Minor questions, otherwise ACK

[TheBlueMatt]

nit, though its not worth changing back - we can leave doc comments on private items just fine, cargo doc has a --document-private-... flag to generate the docs for them, even. That said, it doesn't matter much we don't really use private docs AFAIK, but some devs may in the future.

[tnull]

Ah, fair enough, just thought I'd keep it consistent with visibility.