feat(batch): improve connection pool value

[chenzl25]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

• Related Issue Sysbench performance bottleneck analysis -- point select #13459
• Sysbench shows that RPC connection pool get_by_addr consumes about 3.25% CPU. We can use a Arc value to avoid the expensive Vec clone.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[wenym1]

Rest LGTM.

[BugenZhao]

What about directly letting RpcClient::new_clients return Result<Arc<[Self]>>?

[chenzl25]

RpcClient::new_clients

Sounds good. Both ok to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9768a72) 68.25% compared to head (eec1e00) 68.18%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13669      +/-   ##
==========================================
- Coverage   68.25%   68.18%   -0.07%     
==========================================
  Files        1522     1522              
  Lines      261761   261763       +2     
==========================================
- Hits       178660   178494     -166     
- Misses      83101    83269     +168     

Flag	Coverage Δ
rust	68.18% <100.00%> (-0.07%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.