dns: make ctor private

This ensures one can't instanciate a DNSResolver object by
mistake, but uses the singleton. A separate create static
function is added for cases where a new object is explicitely
needed.

src/common/dns_utils.cpp

@@ -334,6 +334,11 @@ DNSResolver& DNSResolver::instance()
   return *staticInstance;
 }
 
+DNSResolver DNSResolver::create()
+{
+  return DNSResolver();
+}
+
 bool DNSResolver::check_address_syntax(const char *addr)
 {
   // if string doesn't contain a dot, we won't consider it a url for now.