MXS-1220: Implement /sessions/ resource

The /sessions/ resource was not implemented due to changes in the core
polling mechanics. With the new worker thread messaging system, sessions
can be listed in a safe manner.

include/maxscale/session.h

@@ -423,4 +423,13 @@ void session_clear_stmt(MXS_SESSION *session);
  */
 json_t* session_to_json(const MXS_SESSION *session, const char* host);
 
+/**
+ * @brief Convert all sessions to JSON
+ *
+ * @param host Hostname of this server
+ *
+ * @return A JSON array with all sessions
+ */
+json_t* session_list_to_json(const char* host);
+
 MXS_END_DECLS