fix dual-stack superslave when multiple namservers share one ip
Revert "Check that the domain isn't already served by the new master"

This reverts commit 3b797caab6583a6a7452a0eaf45fd7a670b4242c.

Conflicts:
	pdns/packethandler.cc
don't addSuckRequest after supermaster zone creation to avoid simultaneous AXFR for the new zone