feat: scale shards to 1 when there is not enough capacity

[MrCroxx]

What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed. :D

As title.

Checklist

• I have written the necessary rustdoc comments
• I have added the necessary unit tests and integration tests
• I have passed make all (or make fast instead if the old tests are not modified) in my local environment.

Related issues or PRs (optional)

close #764

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer-memory/src/cache.rs	77.77%	4 Missing ⚠️

Files with missing lines	Coverage Δ
foyer-memory/src/generic.rs	91.04% <100.00%> (+0.05%)	⬆️
foyer-memory/src/cache.rs	85.74% <77.77%> (-0.44%)	⬇️