Make backand.go maps smaller (#2380)
Make backand.go maps smaller (#2380)

These maps where all map[x]bool. Change this a map[x]struct{} as this
is smaller and we only use these map to signal "this element exists".

This should preserve a (small) amount of memory.

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
coredns/coredns,miekg/coredns,ii/coredns,johnbelamaric/coredns,ii/coredns,chrisohaver/coredns,miekg/coredns,johnbelamaric/coredns,yongtang/coredns