In a conveyance of real estate the words "mortgage covenants" shall have the full force, meaning and effect of the following words, and shall be applied and construed accordingly: "The grantor covenants with the grantee, his heirs and assigns that he is lawfully seized in fee of the premises, that they are free of all encumbrances, that he has good right to sell and convey the same to the said grantee to hold as aforesaid and that he and his heirs shall and will warrant and defend the same to the said grantee, his heirs and assigns forever, against the lawful claims and demands of all persons."   [PL 1967, c. 377 (NEW).]