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.h

@@ -49,7 +49,7 @@ struct DNSResolverData;
  */
 class DNSResolver
 {
-public:
+private:
 
   /**
    * @brief Constructs an instance of DNSResolver
@@ -58,6 +58,8 @@ public:
    */
   DNSResolver();
 
+public:
+
   /**
    * @brief takes care of freeing C pointers and such
    */
@@ -119,6 +121,13 @@ public:
    */
   static DNSResolver& instance();
 
+  /**
+   * @brief Gets a new instance of DNSResolver
+   *
+   * @return returns a pointer to the new object
+   */
+  static DNSResolver create();
+
 private:
 
   /**