MXS-2314 Add 'cluster' parameter to service

Using the cluster parameter, the servers of a service can be
defined using a monitor.

This change basically only introduces the parameter.

server/core/internal/service.hh

@@ -21,6 +21,8 @@
 
 #include "filter.hh"
 
+class Monitor;
+
 /**
  * @file service.h - MaxScale internal service functions
  */
@@ -103,6 +105,9 @@ public:
     // TODO: Make these private
     mutable std::mutex lock;
 
+    // TODO: Make this private.
+    Monitor*    m_monitor { nullptr }; /**< A possibly associated monitor */
+
 private:
     FilterList  m_filters;          /**< Ordered list of filters */
     std::string m_name;             /**< Name of the service */
@@ -399,5 +404,13 @@ json_t* service_relations_to_server(const SERVER* server, const char* host);
  */
 json_t* service_relations_to_filter(const SFilterDef& filter, const char* host);
 
+/**
+ * @brief Add server to all services associated with a monitor
+ *
+ * @param monitor  A monitor.
+ * @param server   A server.
+ */
+void service_add_server(Monitor* pMonitor, SERVER* pServer);
+
 std::unique_ptr<ResultSet> serviceGetList(void);
 std::unique_ptr<ResultSet> serviceGetListenerList(void);