perf(coordinator): optimize coordinator get task's index

[georgehao]

Purpose or design rationale of this PR

Describe your change. Make sure to answer these three questions: What does this PR do? Why does it do it? How does it do it?

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• build: Changes that affect the build system or external dependencies (example scopes: yarn, eslint, typescript)
• ci: Changes to our CI configuration files and scripts (example scopes: vercel, github, cypress)
• docs: Documentation-only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that doesn't fix a bug, or add a feature, or improves performance
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• test: Adding missing tests or correcting existing tests

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• No, this PR doesn't involve a new deployment, git tag, docker image tag
• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change
• Yes

[codecov]

Codecov Report

Merging #962 (dea5df7) into develop (410f14b) will decrease coverage by 0.17%.
The diff coverage is 0.00%.

❗ Current head dea5df7 differs from pull request most recent head d21ff8e. Consider uploading reports for the commit d21ff8e to get more accurate results

@@             Coverage Diff             @@
##           develop     #962      +/-   ##
===========================================
- Coverage    50.50%   50.33%   -0.17%     
===========================================
  Files           96       96              
  Lines         9960     9993      +33     
===========================================
  Hits          5030     5030              
- Misses        4516     4549      +33     
  Partials       414      414              

Flag	Coverage Δ
coordinator	18.94% <0.00%> (-0.58%)	⬇️

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

Files Changed	Coverage Δ
coordinator/internal/orm/batch.go	0.00% <0.00%> (ø)
coordinator/internal/orm/chunk.go	0.00% <0.00%> (ø)