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What's changed and what's your intention?

Motivation: #1010

This PR:

1. Remove the implementation of SqlQueryHandler for Datanode Instance and DistInstance.
2. "select" is executed in query engine, while other SQLs are executed via a new StatementHandler. When more statements are executed in the form of logical plan, this StatementHandler will be removed eventually.
3. Both Datanode instance and DistInstance implement StatementHandler.

So the SQL execution path is like this:

Frontend Instance still impl the SqlQueryHandler, which is called by servers. Inside the impl:

fn do_query(&self, sql: &str) -> Output {
  let stmt = parse(sql);
  match stmt {
    Statement::Query | Statement::Tql => {
        let plan = plan(stmt);
        self.query_engine.execute(plan)
    }
    _ => {
        self.statement_handler.handle(stmt)
    }
  }
}

The plan is to gradually move all statement execution to logical plan execution. Finally unified to query_engine's execution of logical plan.

The difference between standalone and distributed execution of logical plan should be revealed in different implementation of QueryEngine, which will be in the following PRs.

4. Remove the MySQL server implementation from Datanode.
5. Extract the codes that create logical plan in query engine to a LogicalPlanner, making codes more clear.
6. Other codes clean up.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#1010