refactor: refactor region lease keeper

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

I realized we can't simply retain active leader regions and also need to find out the downgraded leader regions.

1. Refactor find_staled_leader_regions; now, it can find out the downgraded leader regions.
2. Add find_staled_follower_regions, returning upgradable and closable regions.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#2700

[codecov]

Codecov Report

Merging #2704 (99aa8d6) into develop (7e0dcfc) will decrease coverage by 0.33%.
Report is 6 commits behind head on develop.
The diff coverage is 98.28%.

❗ Current head 99aa8d6 differs from pull request most recent head a80462d. Consider uploading reports for the commit a80462d to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2704      +/-   ##
===========================================
- Coverage    85.16%   84.83%   -0.33%     
===========================================
  Files          763      763              
  Lines       122831   123311     +480     
===========================================
+ Hits        104613   104615       +2     
- Misses       18218    18696     +478     

[WenyXu]

@fengjiachun PTAL

[fengjiachun]

LGTM