fix: store 'block_until' in persistent storage

[lemmih]

Summary of changes

Changes introduced in this pull request:

• The Durable Object is clearing out its in-memory state more often than I expected. This PR moves the state into persistent storage.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code
  adheres to the team's
  documentation standards,
• I have added tests that prove my fix is effective or that my feature works
  (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes
  should be reflected in this document.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 10.02%. Comparing base (3dd52e9) to head (dc71b52).

Files with missing lines	Patch %	Lines
src/rate_limiter.rs	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   10.10%   10.02%   -0.08%     
==========================================
  Files          25       25              
  Lines        1277     1287      +10     
==========================================
  Hits          129      129              
- Misses       1148     1158      +10     

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