Parallelise email DNS lookup for new-reg (#1731)
Parallelise email DNS lookup for new-reg (#1731)

* Parallelise email DNS lookup for new-reg

* Remove the 1-case switch

benileo/boulder,jsha/boulder,benileo/boulder,jcjones/boulder,sophie-h/boulder,sophie-h/boulder,bmw/boulder,lucasallan/boulder,letsencrypt/boulder,patf/boulder,jfrazelle/boulder,patf/boulder,benileo/boulder,letsencrypt/boulder,jcjones/boulder,jfrazelle/boulder,jcjones/boulder,jcjones/boulder,jsha/boulder,ZCloud-Firstserver/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,letsencrypt/boulder,sophie-h/boulder,jfrazelle/boulder,lucasallan/boulder,benileo/boulder,lucasallan/boulder,patf/boulder,ZCloud-Firstserver/boulder,ibukanov/boulder,patf/boulder,ZCloud-Firstserver/boulder,bmw/boulder,ibukanov/boulder,ibukanov/boulder,bmw/boulder,jsha/boulder