chore: always obsolete wal to avoid discontinuous log entries

[v0y4g3r]

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What's changed and what's your intention?

This changes the behavior during datanode startup in that it will always obsolete wal log entries to flushed_entry_id in manifest so that if it can tolerate inserts when the entry id span in logstore < flushed_entry_id.

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.

[evenyag]

Mostly LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.24%. Comparing base (20a933e) to head (7adc350).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3789      +/-   ##
==========================================
- Coverage   85.55%   85.24%   -0.32%     
==========================================
  Files         947      947              
  Lines      160481   160476       -5     
==========================================
- Hits       137307   136791     -516     
- Misses      23174    23685     +511