Section 3A. Every officer to whom a warrant to search is issued shall return the same to the court by which it was issued as soon as it has been served and in any event not later than seven days from the date of issuance thereof, with a return of his doings thereon; provided, however, that a justice of the superior court may at any time receive complaints and issue search warrants returnable in seven days before a district court named in such warrant and in that event the officer shall make his return to such district court as directed.