refactor: make MergeTreeMemtable the default choice

[v0y4g3r]

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Refer to a related PR or issue link (optional)

#2804

What's changed and what's your intention?

Though still in experimental stage, we can replace TimeSeriesMemtable with MergeTreeMemtable.

This PR also modifies some of the default config of MergeTreeMemtable.

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 91.11111% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 84.72%. Comparing base (4915786) to head (39938d7).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3430      +/-   ##
==========================================
- Coverage   85.09%   84.72%   -0.38%     
==========================================
  Files         889      890       +1     
  Lines      146519   146093     -426     
==========================================
- Hits       124677   123770     -907     
- Misses      21842    22323     +481     

[evenyag]

LGTM

[fengjiachun]

LGTM