feat: refine either storage

[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

This PR refined the implementation of Either and Selector to make them behave.

Changes:

• Refined Either and Selector.
• Replace EnqueueHandle with WaitHandle with more flexibilities.

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)

#596

[codecov]

Codecov Report

Attention: Patch coverage is 33.64486% with 71 lines in your changes missing coverage. Please review.

Project coverage is 82.43%. Comparing base (3c15f70) to head (58e58ac).

Files	Patch %	Lines
foyer-storage/src/storage/either.rs	0.00%	35 Missing ⚠️
foyer-storage/src/engine.rs	26.47%	25 Missing ⚠️
foyer-storage/src/small/generic.rs	0.00%	9 Missing ⚠️
foyer-storage/src/large/generic.rs	88.88%	1 Missing ⚠️
foyer-storage/src/storage/runtime.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #609      +/-   ##
==========================================
- Coverage   82.72%   82.43%   -0.30%     
==========================================
  Files          62       62              
  Lines        8016     8037      +21     
==========================================
- Hits         6631     6625       -6     
- Misses       1385     1412      +27     

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