Add class Sql_cmd_call

This is a self-containing sub-task of MDEV-10591.
It's done separately, to reduce the patch size for MDEV-10591, to easier reviewing purposes.

Currently SQLCOM_CALL is handled in a huge piece of code in a switch in mysql_execute_command():

case SQLCOM_CALL:

  {

    // around 50 code lines

  }

We'll introduce a new class Sql_cmd_call and move this code to Sql_cmd_call::execute.

After this change, CALL statements will be handled in the same way with other statements using Sql_cmd based implementations:

• ALTER TABLE
• ALTER SEQUENCE
• CHECK
• OPTIMIZE
• REPAIR
• SIGNAL
• RESIGNAL
• GET DIAGNOSTICS
• TRUNCATE

Reasoning:

• MDEV-10591 will need more data (other than LEX::sphead) to execute the query: a pointer to Sp_handler will be needed, to distinguish between calls for standalone vs package procedures. Adding a new member to LEX is not a good idea.
• All statements should eventually be refactored to use the Sql_cmd infrastructure anyway.