fix: do not scale in shards when capacity is not enough, just warn

[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

Sometimes, it is still useful.

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)

fix #768

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer-memory/src/cache.rs	50.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
foyer-memory/src/generic.rs	90.58% <ø> (-0.47%)	⬇️
foyer-memory/src/cache.rs	84.59% <50.00%> (-1.16%)	⬇️