refactor: change the receivers of merge tree components

[v0y4g3r]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Change the receivers of merge tree components' read methods to &self instead of &mut self so that Partition::read does not need to acquire write lock.

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.

Refer to a related PR or issue link (optional)

• Tracking issue for the memtable improvement #2804

[codecov]

Codecov Report

Attention: Patch coverage is 96.03960% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.29%. Comparing base (e481f07) to head (ae54a37).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3378      +/-   ##
==========================================
- Coverage   85.67%   85.29%   -0.38%     
==========================================
  Files         893      893              
  Lines      147247   147207      -40     
==========================================
- Hits       126152   125566     -586     
- Misses      21095    21641     +546     

[evenyag]

LGTM

[fengjiachun]

LGTM