Allow subclasses of BatchAntiSpoof to override the wfWaitForSlaves() call

So they can wait for a different database or cluster.

Change-Id: I1e90af4d87e4a38cf7ba72268542aeb2ac91d9dd
(cherry picked from commit a20060c7a5b58f430e8d23eb0b5d3c1cf76519e0)