feat: expose rocksdb background jobs option

[jiacai2050]

Which issue does this PR close?

Closes #

Rationale for this change

Max background jobs is hard-code, expose an option to configure this.

What changes are included in this PR?

• Add new options for background jobs.
• Add new http endpoint for debugging GET /debug/stats, this require following config:

[analytic.wal.data_namespace]
enable_statistics = true
max_background_jobs = 8

Are there any user-facing changes?

No

How does this change test

No need.

[codecov-commenter]

Codecov Report

Merging #812 (6b47fb8) into main (d9b7d15) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 6b47fb8 differs from pull request most recent head 4ddb0ac. Consider uploading reports for the commit 4ddb0ac to get more accurate results

@@            Coverage Diff             @@
##             main     #812      +/-   ##
==========================================
- Coverage   67.99%   67.99%   -0.01%     
==========================================
  Files         299      299              
  Lines       47174    47173       -1     
==========================================
- Hits        32077    32075       -2     
- Misses      15097    15098       +1     

Impacted Files	Coverage Δ
analytic_engine/src/lib.rs	98.86% <100.00%> (+0.05%)	⬆️
analytic_engine/src/manifest/details.rs	95.62% <100.00%> (-0.01%)	⬇️
analytic_engine/src/setup.rs	56.00% <100.00%> (ø)
wal/src/rocks_impl/manager.rs	91.06% <100.00%> (-0.07%)	⬇️

... and 1 file with indirect coverage changes

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[ShiKaiWi]

LGTM