refactor(storage): remove arc in shared buffer event sender

[wenym1]

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What's changed and what's your intention?

As title. Our current code use an Arc to wrap the shared buffer event sender, which is of type mpsc::UnboundedSender. But it seems that the type support Clone trait and it implements reference counting internally. Therefore, we don't need the Arc any more and we can clone from the raw sender.

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

[hzxa21]

I am just about to open a PR for this lol

[codecov]

Codecov Report

Merging #3518 (e4106e0) into main (8203ac7) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3518      +/-   ##
==========================================
- Coverage   74.46%   74.46%   -0.01%     
==========================================
  Files         769      769              
  Lines      108054   108054              
==========================================
- Hits        80466    80462       -4     
- Misses      27588    27592       +4     

Flag	Coverage Δ
rust	74.46% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/storage/src/hummock/local_version_manager.rs	79.15% <100.00%> (-0.13%)	⬇️
src/storage/src/hummock/shared_buffer/mod.rs	90.58% <100.00%> (ø)
...e/src/hummock/shared_buffer/shared_buffer_batch.rs	93.35% <100.00%> (ø)
src/meta/src/hummock/mock_hummock_meta_client.rs	40.56% <0.00%> (-0.95%)	⬇️
src/meta/src/manager/id.rs	94.94% <0.00%> (-0.57%)	⬇️
src/meta/src/barrier/mod.rs	81.19% <0.00%> (-0.21%)	⬇️
src/common/src/types/ordered_float.rs	24.70% <0.00%> (-0.20%)	⬇️
src/connector/src/filesystem/file_common.rs	80.80% <0.00%> (+0.44%)	⬆️

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