fix: correctly generate sequences when the value is pre-existed

[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?

Sometimes the value in the kvbackend that is used to store the sequence could be pre-existed, even before the sequence has been built. If the value is smaller than the set initial of sequence, the sequence will start from the (smaller) value, which violates the intuition that sequence must start from initial. This PR fixes that.

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

Attention: Patch coverage is 94.44444% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 84.94%. Comparing base (904484b) to head (b57ffb5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3502      +/-   ##
==========================================
- Coverage   85.31%   84.94%   -0.38%     
==========================================
  Files         903      903              
  Lines      149704   149782      +78     
==========================================
- Hits       127727   127231     -496     
- Misses      21977    22551     +574     

[evenyag]

LGTM

[fengjiachun]

LGTM