refactor!: Renames the new memtable to PartitionTreeMemtable

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

• Tracking issue for the memtable improvement #2804

What's changed and what's your intention?

This PR renames the new memtable (MergeTreeMemtable) to PartitionTreeMemtable. All experimental configurations for the new memtable will be changed to use partition_tree.

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.

[fengjiachun]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 91.48936% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 84.98%. Comparing base (6377982) to head (f9d5a20).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3547      +/-   ##
==========================================
- Coverage   85.36%   84.98%   -0.39%     
==========================================
  Files         910      910              
  Lines      152030   152034       +4     
==========================================
- Hits       129780   129202     -578     
- Misses      22250    22832     +582     

[v0y4g3r]

LGTM