Refined the class Sql_cmd_dml as a subclass of Sql_cmd for processing
of SQL DML statements. Subclasses of Sql_cmd_dml are created for these
classes of SQL statements:

- SELECT
- INSERT ... VALUES, INSERT ... SELECT
- UPDATE (both single-table and multi-table variation)
- DELETE (both single-table and multi-table variation)
- DO
- CALL

Refactored processing of INSERT, UPDATE and DELETE statements for clear
separation of preparation and execution code, as well as simplifying
the execution logic.

After this refactoring, most DML statements go through a fixed set of
processes, each implemented using member functions, most of them virtual.
SET statements and CREATE TABLE ... SELECT statements are still processed
by handle_query() after this worklog, however.

Preparing a statement goes through these processes:

- precheck()
- open_tables_for_query()
- resolve_var_assignments()
- prepare_inner() (the actual preparation code)
- cleanup()

Executing a statement goes through these processes:

- set_statement_timer()
- preparation (if not already done, otherwise precheck() and open_tables())
- run_before_dml_hook() (if data change statement)
- push_internal_handler() (if data change statement)
- lock_tables()
- query_cache.store_query()
- execute_inner() (the actual optimization and execution code)
- pop_internal_handler()
- cleanup()
- reset_statement_timer()

Query_result objects are changed so that they are created for
the duration of the statement object.