During a table-rebuilding online ALTER TABLE, if
dict_index_t::remove_instant() was invoked on the source table
(because it became empty), we would inadvertently change the way
how log records are written and parsed. We must keep the online_log
format unchanged throughout the whole table-rebuilding operation.

dict_col_t::def_t: Name the type of dict_col_t::def_val.

rec_get_n_add_field_len(), rec_set_n_add_field(): Define globally,
because these will be needed in row_log_table_low().

rec_init_offsets_temp(), rec_init_offsets_comp_ordinary(): Add
the parameter def_val for explicitly passing the default values
of the instantly added columns of the source table, so that
dict_index_t::instant_field_value() will not be called during
row_log_table_apply(). This allows us to consistently parse the
online_log records, even if the source table was converted
to the canonical non-instant format during the rebuild operation.

row_log_t::non_core_fields[]: The default values of the
instantly added columns on the source table; copied
during ha_innobase::prepare_inplace_alter_table()
while the table is exclusively locked.

row_log_t::instant_field_value(): Accessor to non_core_fields[],
analogous to dict_index_t::instant_field_value().

row_log_table_low(): Add fake_extra_size bytes to the record
header if the source table was converted to the canonical format
during the operation.

row_log_allocate(): Initialize row_log_t::non_core_fields.