Implement server-side cursors by handling COM_STMT_FETCH command.
The client indicates that it wants to use cursor
by setting a flag in COM_STMT_EXECUTE
Please refer to https://dev.mysql.com/doc/internals/en/com-stmt-execute.html
Subsequently, the client acquires result rows repeatedly by COM_STMT_FETCH,
which will carry stmt-id and fetch size.
Please refer to https://dev.mysql.com/doc/internals/en/com-stmt-fetch.html
This commit only support forward-only, read-only cursor
