planner: refactor the IsolationReadEngines variable for plan cache

[Reminiscent]

What problem does this PR solve?

Issue Number: ref #36598
ref #36770
Problem Summary: planner: refactor the IsolationReadEngines variable for plan cache

What is changed and how it works?

1. Unimport the IsolationReadEngines in variables.
2. Use GetIsolationReadEngines to access the IsolationReadEngines values.
3. Use the GetAvailableIsolationReadEngines4Plan function to get the available isolation read engines. Not delete the tif lash engine first and add it later.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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