fix: run purge jobs in another scheduler

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

• Optimize compaction strategy to avoid merging SST files without overlapping time ranges #3416

What's changed and what's your intention?

This PR creates a new scheduler for purge jobs instead of reusing the global scheduler. It avoids purge jobs blocking other background jobs, especially flush jobs.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.03%. Comparing base (4a5bb69) to head (e5ca70c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3621      +/-   ##
==========================================
- Coverage   85.38%   85.03%   -0.36%     
==========================================
  Files         932      932              
  Lines      155596   155605       +9     
==========================================
- Hits       132857   132319     -538     
- Misses      22739    23286     +547     

[killme2008]

LGTM 👍

[v0y4g3r]

LGTM