sql: make FOR EACH ROW clause mandatory in trigger definition

Before this patch, it was possible to create a trigger without FOR EACH
ROW clause, for example:

CREATE TRIGGER trg AFTER DELETE ON tbl BEGIN ; END;

In ANSI SQL, if trigger-timing-clause is not specified, FOR EACH
STATEMENT is used. Tarantool, however, did not support FOR EACH
STATEMENT and assumed FOR EACH ROW. This could break future
applications, once FOR EACH STATEMENT is added.

Thus, make FOR EACH ROW clause mandatory. Update tests.

No docs ticket since there is no docs for this feature yet :/ -
will document the fixed behaviour right away.