Remove Send + Sync bounds when no-std

[Fedeparma74]

Summary

This PR addresses #4176

Details

• Removed unnecessary Send and Sync bounds in the lightning-background-processor crate and the async KVStore trait, which were causing compilation issues when building without the std feature.
• The changes ensure better compatibility for no_std environments while preserving async functionality.

Rationale

These bounds were preventing the code from compiling in async, no-std configurations. By relaxing them, the async background processor and async persistence can now be used in embedded or WASM environments without requiring Send/Sync.

[ldk-reviews-bot]

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.86%. Comparing base (afb80e5) to head (8e4b8e4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-background-processor/src/lib.rs	44.44%	5 Missing ⚠️
lightning/src/util/test_utils.rs	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4184      +/-   ##
==========================================
- Coverage   88.86%   88.86%   -0.01%     
==========================================
  Files         180      180              
  Lines      137869   137863       -6     
  Branches   137869   137863       -6     
==========================================
- Hits       122520   122511       -9     
- Misses      12539    12546       +7     
+ Partials     2810     2806       -4     

Flag	Coverage Δ
fuzzing	20.85% <5.88%> (+<0.01%)	⬆️
tests	88.70% <69.23%> (-0.01%)	⬇️

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[tnull]

CI is broken, but I just confirmed that the current state works with LDK Node.

[tnull]

This change made rustfmt unhappy.

[Fedeparma74]

Weird, as I'm using rustfmt as well. Probably two different versions. Pushed a fix :)

[valentinewallace]

Looks like CI is still sad :/

[TheBlueMatt]

Sadly needs rebase now, but should be pretty trivial, also indeed the test kvstore utils need updating to the new API. I'd like to include this in 0.2, but we're rapidly running out of time for that.

[TheBlueMatt]

Would you mind rebasing on current git, rather than merging? We require each PR to be a freestanding list of commits, with no merges in them.

[TheBlueMatt]

Feel free to squash the last two fixup commits into the second commit - we try to require that each commit individually builds and passes tests to ensure git bisect is usable later.

[Fedeparma74]

I hope everything looks good now! Sorry for the extra work earlier, and thanks for your guidance! :)

[TheBlueMatt]

Thanks! In the future it would be nice to get more context in the commit message but this time we'll let it slide :). Gonna go ahead and land this since @tnull said it doesn't break ldk-node and we want to get this in.

[TheBlueMatt]

Backported in #4185