refactor: make region manifest checkpoint ran in background

[MichaelScofield]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Region manifest checkpoint currently requires many object store reads and writes. The checkpoint is performed synchronously in the region manifest update. When the object store is slow, it could easily cause the region manifest update method executes too long. This PR makes the checkpoint ran in the background (performed asynchronously) to solve the problem.

This PR also makes some changes to how the checkpoint is done. Originally the current checkpoint is the last checkpoint merged with all the incremental files. This process has to scan the object store. However, since the checkpoint is actually a snapshot of the current region manifest, which is already present in the memory, stick to the old ways seems unnecessary. So this PR directly use the region manifest snapshot to make current checkpoint, and delete the obsolete incremental files.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 93.42105% with 10 lines in your changes missing coverage. Please review.

Project coverage is 84.67%. Comparing base (e982d2e) to head (db7ca71).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4133      +/-   ##
==========================================
- Coverage   85.39%   84.67%   -0.72%     
==========================================
  Files         994     1019      +25     
  Lines      173994   178741    +4747     
==========================================
+ Hits       148580   151351    +2771     
- Misses      25414    27390    +1976     

[MichaelScofield]

@evenyag @WenyXu @killme2008 PTAL

[killme2008]

LGTM. It will help a lot when writing.

[MichaelScofield]

@evenyag PTAL