WL#5070: Prepare Sql_cmd class for addition of new statement classes

Initial part of WL#5047, which is about transforming SQL execution into an 
object hierarchy with methods for preparation and execution.

The existing Sql_statement class (defined in sql_lex.h) will be modified as 
follows:

 - The class is renamed to Sql_cmd so that it is not confused with Statement
   and Prepared_statement.

 - Remove the m_lex member from the base class (it should not be necessary,
   and the LEX object should eventually go away.

 - Add a method sql_command_code() that is used to access the legacy SQL command
   code of the statement.

The Sql_cmd class, including the enum_sql_command definition, is put into 
a separate file sql/sql_cmd.h.

No HLS is required for this WL.

New methods on Sql_cmd:

virtual enum_sql_command sql_command_code() const;

This method will be implemented for each derived class of Sql_cmd to 
return the legacy SQL command code associated with the statement.