Problem:- If we create table field with dynamic default value then that
 field always gets NULL value.

Analyze:- This is because in fill_record we simple continue at Invisible
 column because we though that share->default_values(default value is
 always copied into table->record[0] before insert) will have a default
 value for them(which is true for constant defaults , but not for dynamic
 defaults).

Solution:- We simple set all_fields_have_value to null , and this will
make call to update_default_fields (in the case of dynamic default), And
default expr will be evaluted and value will be set in field.

server crash in JOIN::fix_all_splittings_in_plan

Cost formulas must take into account the case when a splittable table
has now rows.

Only give warning if warnings > 2, as there is no plan to change
the current behavior.

- When adding LEX_CSTRING to String, we are now checking that
  string is \0 terminated (as normally LEX_CSTRING should be
  usable for printf(). In the cases when one wants to avoid the
  checking one can use String->append(ptr, length) instead of just
  String->append(LEX_CSTRING*)

This preserves const str for constant strings

Other things
- A few variables where changed from LEX_STRING to LEX_CSTRING
- Incident_log_event::Incident_log_event and record_incident where
  changed to take LEX_CSTRING* as an argument instead of LEX_STRING

This is to make it easier to use the
create_mysqld_error_find_printf_error tool to find wrong print

Rename was done as the old 'name' hide the original item name.