1. add StatementContext, and PlannerContext is renamed to CascadsContext. CascadsContext belong to a StatementContext, and StatementContext belong to a ConnectionContext, and the lifecycle increases in turn. StatementContext can wrap some statement's lifecycle-related state, such as ExpressionId, TableLock. MemoTestUtil can simplify create a CascadesContext and Memo for test.
2. add PlanPreprocessor to process parsed logical plan before copy into memo. and add a PlanPostprocessor to process physical plan after copy out from memo.
3. utilize PlanPreprocessor to process SET_VAR hint, the class is EliminateLogicalSelectHint
4. pass the limit clause in regression test case, in set_var.groovy
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