WL#6891 : Define and reimplement STRICT MODE

IGNORE Reimplementation :
This worklog reimplements IGNORE. In the previous implementation,
errors were downgraded to warnings inside THD::raise_condition()
based on SELECT_LEX::no_error. This patch changes it to use an
Internal_error_handler activated just for statements which support
IGNORE keyword to downgrade specific errors to warnings.

Bugs Fixed by this worklog :

- Bug#11744960 : INSERT IGNORE SHOULD RETURN WARNINGS
- Bug#11752648 : MULTI-DELETE IGNORE DOES NOT REPORT WARNINGS
- Bug#16522924 : UPDATE TRIGGER INVOKED WHEN UPDATE IGNORE
                 MEANS THAT NO UPDATE IS PERFORMED
- Bug#16860715 : ASSERT IN PROTOCOL::END_STATEMENT DURING DELETE
                 IGNORE
- Bug#16860829 : ASSERT IN DIAGNOSTICS_AREA::SET_ERROR_STATUS
- Bug#17550423 : DELETE IGNORE GIVES INCORRECT RESULT WITH FOREIGN
                 KEY FOR PARENT TABLE


STRICT Mode Reimplementation :
This worklog reimplements STRICT mode. In the previous implementation
When STRICT MODE was enabled, warning were upgraded to errors inside
THD::raise_condition() based on THD::abort_on_warning flag.
This patch uses an Internal_error_handler activated just for
STRICT MODE to do the upgrade of specific warnings to errors.

Bugs Fixed by this worklog :

- BUG#11751889: TRIGGERS OVERRIDE STRICT SQL_MODE
- Bug#16976939: FIX ERROR MESSAGE ON DUPLICATE INDEX CREATION AND
                STRICT MODE
- BUG#18526888: STRICT MODE DOES NOT APPLY TO MULTI DELETE STATEMENT