Inbound user_channel_id randomization follow-up

[tnull]

This is a follow-up to #1790, which addresses two issues:

1. While the randomized user_channel_id was indeed set in Channel::new_from_req, it was actually overridden with a 0 value just a few lines below. In the first commit, we address this oversight.
2. We now mention as of which version the user can expect the user_channel_id to be randomized, addressing Randomize user_channel_id for inbound channels #1790 (comment).

[codecov-commenter]

Codecov Report

Base: 90.68% // Head: 91.51% // Increases project coverage by +0.82% 🎉

Coverage data is based on head (7f6713c) compared to base (8d8ee55).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1855      +/-   ##
==========================================
+ Coverage   90.68%   91.51%   +0.82%     
==========================================
  Files          89       90       +1     
  Lines       47947    53434    +5487     
  Branches    47947    53434    +5487     
==========================================
+ Hits        43481    48900    +5419     
- Misses       4466     4534      +68     

Impacted Files	Coverage Δ
lightning/src/onion_message/functional_tests.rs	95.83% <ø> (ø)
lightning/src/util/events.rs	25.25% <ø> (ø)
lightning/src/ln/channelmanager.rs	87.62% <100.00%> (+2.55%)	⬆️
lightning/src/ln/functional_test_utils.rs	95.96% <100.00%> (+2.49%)	⬆️
lightning-net-tokio/src/lib.rs	76.73% <0.00%> (-0.31%)	⬇️
lightning/src/ln/reorg_tests.rs	100.00% <0.00%> (ø)
lightning/src/ln/reload_tests.rs	95.24% <0.00%> (ø)
lightning/src/ln/chanmon_update_fail_tests.rs	97.72% <0.00%> (+0.05%)	⬆️
lightning/src/ln/payment_tests.rs	99.34% <0.00%> (+0.45%)	⬆️
lightning/src/ln/onion_route_tests.rs	98.35% <0.00%> (+0.80%)	⬆️
... and 7 more

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[valentinewallace]

I checked that we always set inbound channel user_ids to a random value by asserting that it's non-0 in channel.funding_created and channel.funding_signed and ensuring tests pass

[tnull]

I checked that we always set inbound channel user_ids to a random value by asserting that it's non-0 in channel.funding_created and channel.funding_signed and ensuring tests pass

Yeah, I'm a little unsure whether to include a regression test checking for non-0 as it technically is indeterministic. Then again, it's veeery unlikely we'd ever hit the 0 in a u128. Any preferences?

[valentinewallace]

No strong preference. I'm fine rolling the dice on that, though, if we think a regression test would be good.

[tnull]

No strong preference. I'm fine rolling the dice on that, though, if we think a regression test would be good.

Yeah, after having it written out it seems kinda obvious that it's fine to take the chance 😅
Added some asserts with cdb6c3b.

[TheBlueMatt]

Grrr, good catch. Feel free to squash.

[tnull]

Grrr, good catch. Feel free to squash.

Squashed without further changes.