middleware/proxy: remove singleinflight from dns (#717)

Singleinflight interferes with the health checking of upstream. If an
upstream would fail, singleinflight would mirror that error to to other
proxy *iff* multple identical queries would be inflight. This would lead
to marking *all* upstreams as bad, essentially collapsing multiple
upstreams into a SPOF. Clearly not what we want.

Singleinflight does have some nice properties, but I've opted to rip it
out entirely. Caching should almost (but not quite) as good.

Added a test case in test that uses 3 CoreDNS instances to reflect the
setup from #715.

Found another bug as well, where (when the policy would be nil), we
would always Spray even though we've found a healthy host.

middleware/proxy/upstream.go

@@ -328,6 +328,9 @@ func (u *staticUpstream) Select() *UpstreamHost {
 
 	if u.Policy == nil {
 		h := (&Random{}).Select(pool)
+		if h != nil {
+			return h
+		}
 		if h == nil && u.Spray == nil {
 			return nil
 		}