[Refactor] Unify the local cache engine used by data cache.

[trueeyu]

Why I'm doing:

Before the PR, the block cache used the StarCache engine, and the page cache used the LRUCache engine. After the PR, DataCache will only use one local cache engine—either StarCache or LRUCache—so that memory space can be unified managed.

What I'm doing:

• Unify the cache engine used by data cache
• If datacache_enable=false, page cache and block cache is all disabled
• Set default value of config::datacache_mem_size to 20%
• TODO: doc and mertrics will be modified in next pr.

What type of PR is this:

• [ ] BugFix
• [ ] Feature
• [ ] Enhancement
• [x] Refactor
• [ ] UT
• [ ] Doc
• [ ] Tool

Does this PR entail a change in behavior?

• [x] 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
• [x] 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

Bugfix cherry-pick branch check:

• [x] I have checked the version labels which the pr will be auto-backported to the target branch
  • [ ] 3.5
  • [ ] 3.4
  • [ ] 3.3