refactor: refine small object disk cache

[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

FYI: #759

Lock Order:

load (async set cache, not good):

                                                                  |------------ requires async mutex -------------|
 lock(R) bloom filter => unlock(R) bloom filter => lock(R) set => lock(e) set cache => load => unlock(e) set cache => unlock(r) set

load (sync set cache, good):

 lock(R) bloom filter => unlock(R) bloom filter => lock(R) set => lock(e) set cache => unlock(e) set cache => load => lock(e) set cache => unlock(e) set cache => unlock(r) set

update:

 lock(W) set => lock(e) set cache => invalid set cache => unlock(e) set cache => update set => lock(w) bloom filter => unlock(w) bloom filter => unlock(w) set

Checklist

• [x] I have written the necessary rustdoc comments
• [x] I have added the necessary unit tests and integration tests
• [x] 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 #759

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.62264% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer-storage/src/small/set_manager.rs	92.18%	5 Missing :warning:
foyer-storage/src/store.rs	42.85%	4 Missing :warning:
foyer-storage/src/small/generic.rs	66.66%	2 Missing :warning:

Files with missing lines	Coverage Δ
foyer-storage/src/small/flusher.rs	85.00% <100.00%> (-0.30%)	:arrow_down:
foyer-storage/src/small/set.rs	94.15% <ø> (-0.02%)	:arrow_down:
foyer-storage/src/small/set_cache.rs	100.00% <100.00%> (ø)
foyer-storage/src/small/generic.rs	75.14% <66.66%> (+0.52%)	:arrow_up:
foyer-storage/src/store.rs	85.33% <42.85%> (-0.73%)	:arrow_down:
foyer-storage/src/small/set_manager.rs	90.28% <92.18%> (+1.96%)	:arrow_up: