v2.3: AppendVec::new_from_storage() does sanitization if len and capacity do not match (backport of #6803)

[mergify]

Problem

Minimized snapshots created with older versions of ledger-tool can have storages with a capacity (file size) larger than their len (number of bytes) (see #6797 for an example). This is no longer an issue in master, as shrink (which is used to write the new, minimized storages) now sets the correct file size for new storages.

After merging #6552, it was no longer possible to open those snapshots with storages that have mismatched capacity and len.

Summary of Changes

At startup when rebuilding storages, if the capacity and len mismatch, fall back to the old/slow impl that does the full sanitization of the storage file.

Fixes #6797

Justification to Backport

Since #6552 was backported to v2.3, the bug also exists on v2.3 (and should be fixed there too).

---

This is an automatic backport of pull request #6803 done by [Mergify](https://mergify.com).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 82.8%. Comparing base (292b33f) to head (6d0d07c).
Report is 1 commits behind head on v2.3.

Additional details and impacted files

@@            Coverage Diff            @@
##             v2.3    #6838     +/-   ##
=========================================
- Coverage    82.8%    82.8%   -0.1%     
=========================================
  Files         850      850             
  Lines      379636   379646     +10     
=========================================
- Hits       314650   314587     -63     
- Misses      64986    65059     +73     

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