Fuzz reloading with a stale monitor in chanmon_consistency

[TheBlueMatt]

Now that we are gearing up to support fully async monitor storage, we really need to fuzz monitor updates not completing before a reload, which we do here in the chanmon_consistency fuzzer.

While there are more parts to async monitor updating that we need to fuzz, this at least gets us started by having basic async restart cases handled. In the future, we should extend this to make sure some basic properties (eg claim/balance consistency) remain true through chanmon_consistency runs.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.65%. Comparing base (1d0c6c6) to head (2a0c900). Report is 15 commits behind head on main.

Files	Patch %	Lines
lightning/src/ln/chanmon_update_fail_tests.rs	97.46%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3113 +/- ## ========================================== + Coverage 89.84% 90.65% +0.80% ========================================== Files 119 119 Lines 97811 102825 +5014 Branches 97811 102825 +5014 ========================================== + Hits 87883 93217 +5334 + Misses 7364 7108 -256 + Partials 2564 2500 -64 ```

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[TheBlueMatt]

Addressed feedback.

[tnull]

Addressed feedback.

Unfortunately the new test now reliably fails:

thread 'ln::chanmon_update_fail_tests::test_sync_async_persist_doesnt_hang' panicked at 'explicit panic', lightning/src/ln/chanmon_update_fail_tests.rs:3410:14

[TheBlueMatt]

Ugh, sorry, pushed right before bed last night and didn't test enough :/

[valentinewallace]

LGTM, feel free to squash IMO.

[TheBlueMatt]

Squashed without further changes.