Cleanups and tests (#272)

For some reasons there was a dnsserver/middleware.go that defined
the middleware handlers. This code was a repeat from
middleware/middleware.go. Removed dnsserver/middleware.go and replaced
all uses of dnsserver.Middleware with middleware.Middleware.

Added dnsserver/address_test.go to test the zone normalization (and to
improve the test coverage). The deleted file will also improve the test
coverage :)

core/dnsserver/address.go

@@ -18,6 +18,8 @@ func (z zoneAddr) String() string { return z.Zone + ":" + z.Port }
 
 // normalizeZone parses an zone string into a structured format with separate
 // host, and port portions, as well as the original input string.
+//
+// TODO(miek): possibly move this to middleware/normalize.go
 func normalizeZone(str string) (zoneAddr, error) {
 	var err error