middleware/secondary: add jitter (#841)
middleware/secondary: add jitter (#841)

randomize the times when doing transfers and checks.

Fixes #840
ii/coredns,chrisohaver/coredns,johnbelamaric/coredns,jremond/coredns,yongtang/coredns,miekg/coredns,coredns/coredns,jremond/coredns,ii/coredns,johnbelamaric/coredns,miekg/coredns