refactor: rename with_buffer_threshold to with_buffer_pool_size

[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 #726

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer/src/hybrid/builder.rs	20.00%	8 Missing ⚠️
foyer-storage/src/store.rs	50.00%	4 Missing ⚠️

Files with missing lines	Coverage Δ
foyer-storage/src/store.rs	73.90% <50.00%> (-0.83%)	⬇️
foyer/src/hybrid/builder.rs	92.82% <20.00%> (-3.67%)	⬇️