MDEV-10164: Add support for TRIGGERS that fire on multiple events

Added capability to create a trigger associated with several trigger
events. For this goal, the syntax of the CREATE TRIGGER statement
was extended to support the syntax structure { event [ OR ... ] }
for the `trigger_event` clause. Since one trigger will be able to
handle several events it should be provided a way to determine what
kind of event is handled on execution of a trigger. For this goal
support of the clauses INSERTING, UPDATING , DELETING was added by
this patch. These clauses can be used inside a trigger body to detect
what kind of trigger action is currently processed using the following
boilerplate:
  IF INSERTING THEN ...
  ELSIF UPDATING THEN ...
  ELSIF DELETING THEN ...
In case one of the clauses INSERTING, UPDATING, DELETING specified in
a trigger's body not matched with a trigger event type, the error
ER_INCOMPATIBLE_EVENT_FLAG is emitted.

After this patch be pushed, one Trigger object will be associated with
several trigger events. It means that the array
  Table_triggers_list::triggers
can contain several pointers to the same Trigger object in array members
corresponding to different events. Moreover, support of several trigger
events for the same trigger requires that the data members `next` and
`action_order` of the Trigger class be converted to arrays to store
relating information per trigger event base.

Ability to specify the same trigger for different event types results in
necessity to handle invalid cases on execution of the multi-event
trigger, when the OLD or NEW qualifiers doesn't match a current event
type against that the trigger is run. The clause OLD should produces
the NULL value for INSERT event, whereas the clause NEW should produce
the NULL value for DELETE event.

sql/sql_insert.cc

@@ -738,7 +738,7 @@ bool mysql_insert(THD *thd, TABLE_LIST *table_list,
   Name_resolution_context_state ctx_state;
   SELECT_LEX *returning= thd->lex->has_returning() ? thd->lex->returning() : 0;
   unsigned char *readbuff= NULL;
-
+  Running_stmt_guard guard(thd, active_dml_stmt::INSERTING_STMT);
 #ifndef EMBEDDED_LIBRARY
   char *query= thd->query();
   /*