MXS-1220: Add Last-Modified and ETag headers

The resource system now tracks both the time when a resource was last
modified and the revision number of the resource. This allows working
Last-Modified and ETag headers to be generated by the REST API.

The If-Modified-Since and If-None-Match request headers are not yet
processed and using them will always return the resource instead of a 304
Not Modified response.

server/core/maxscale/resource.hh

@@ -62,8 +62,8 @@ private:
 
     bool matching_variable_path(const std::string& path, const std::string& target) const;
 
-    ResourceCallback m_cb; /**< Resource handler callback */
-    std::deque<std::string>    m_path; /**< Path components */
+    ResourceCallback        m_cb;   /**< Resource handler callback */
+    std::deque<std::string> m_path; /**< Path components */
 };
 
 /**