For ROW_FORMAT=COMPACT and ROW_FORMAT=DYNAMIC records that contain
fields that have been inserted after columns were instantly added
(the record is tagged REC_STATUS_COLUMNS_ADDED), encode the
number of added columns - 1 in the record header.

rec_get_n_add_field_len): Determine the encoded size of the added fields.

rec_get_n_add_field(): Determine the number of added fields.

rec_set_n_add_field(): Set the number of added fields.