Add Field::tmp_engine_column_type()

There are two similar pieces of the code not friendly to user defined plugins in

• opt_subselect.cc:

  if (field->flags & BLOB_FLAG)

  recinfo->type= FIELD_BLOB;

  else if (use_packed_rows &&

  field->real_type() == MYSQL_TYPE_STRING &&

  length >= MIN_STRING_LENGTH_TO_PACK_ROWS)

  recinfo->type=FIELD_SKIP_ENDSPACE;

  else

  recinfo->type=FIELD_NORMAL;

• sql_select.cc:

  if (field->flags & BLOB_FLAG)

  recinfo->type= FIELD_BLOB;

  else if (use_packed_rows &&

  field->real_type() == MYSQL_TYPE_STRING &&

  length >= MIN_STRING_LENGTH_TO_PACK_ROWS)

  recinfo->type= FIELD_SKIP_ENDSPACE;

  else if (field->real_type() == MYSQL_TYPE_VARCHAR)

  recinfo->type= FIELD_VARCHAR;

  else

  recinfo->type= FIELD_NORMAL;

We'll split this code into a new virtual method in Field:

virtual en_fieldtype tmp_engine_column_type(bool use_packed_rows) const;

so user data types will be able to redefine behaviour as they wish.

Note, originally these two pieces were equal, but then the sql_select.cc version was later extended to handle MYSQL_TYPE_VARCHAR in a special way.
According to SergeP, these two pieces were indended to be equal. So there is probably a bug in the opt_subselect.cc version. This change will fix this problem.