[Enhancement] Support remove immutable memtable for cloud native pindex (backport #54178)

[mergify]

Why I'm doing:

Cloud native persistent index memtable support multiple memtables include one mutable memtable and multiple immutable memtables. However, multiple memtables will use more memory and generate many small sst files which increase read cost. Apart from that, flushing the memtable is not the bottleneck of the apply process, so having a single memtable is more appropriate.

What I'm doing:

Support single memtable for cloud native persistent index.

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr