Optimize String processing in Protocol::send()

Under terms of MDEV-10306 we changed a few Item_func descendants to return the result in the String passed as a parameter to val_str() instead of the class member such as tmp_value. This fixed incorrect results, but caused more my_alloc / my_free.

Consider this script:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a VARCHAR(500));

INSERT INTO t1  VALUES (REPEAT('a',500)),(REPEAT('b',500));

SELECT HEX(a) FROM t1;

This is an except from Protocol::send_result_set_row:

bool Protocol::send_result_set_row(List<Item> *row_items)

{

  char buffer[MAX_FIELD_WIDTH];

  String str_buffer(buffer, sizeof (buffer), &my_charset_bin);

  List_iterator_fast<Item> it(*row_items);

  ...

  for (Item *item= it++; item; item= it++)

  {

    if (item->send(this, &str_buffer))

    ...

  }

}

Notice, the local buffer str_buffer is shared between columns of the same record, but it is not shared between multiple records of the same result set.

The buffer str_buffer is recursively passed to Item_func_hex::val_str_ascii(), which has to do alloc(1001) on every row, because the result does not fit into
MAX_FIELD_WIDTH bytes originally available in str_buffer.

Under terms of this task we will:

• Add a new String member in Protocol, e.g. Protocol::tmp_value.
• Fix Protocol::send_result_set_row() to use this new member instead of the local buffer

This will allow to share the memory allocated on the first row for the consequent rows.