Update KVStore and KVStoreSync arguments to being owned

[joostjager]

This avoids the need for async KVStore implementations to copy data.

Preparation for #3931

[ldk-reviews-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.90%. Comparing base (664511b) to head (23a1eb7).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/util/persist.rs	66.66%	2 Missing and 1 partial ⚠️
lightning/src/util/test_utils.rs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #3974 +/- ##
==========================================
- Coverage 88.91% 88.90% -0.01% 
==========================================
Files 174 174 Lines 124232 124233 +1 Branches 124232 124233 +1 ==========================================
- Hits 110455 110450 -5 - Misses 11301 11304 +3 - Partials 2476 2479 +3 

Flag	Coverage Δ
fuzzing	22.61% <0.00%> (ø)
tests	88.73% <89.47%> (-0.01%)	⬇️

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[ldk-reviews-bot]

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[joostjager]

Post rebase diff: https://github.com/lightningdevkit/rust-lightning/compare/daf19e3aa4d582a9acc195bdea4f86a8d855f047..d37c59567b66d99a2f80feadc2dc0f1693085931

[tnull]

Switch to Display looks good, but this is pending on the other discussion.

[joostjager]

Updated PR according to offline discussion outcome. Only make the data to write owned for both sync and async.

[tnull]

LGTM

[joostjager]

Going to land this to unblock the follow up. Approach has been discussed wider offline, and PR can be checked post-merge @TheBlueMatt.