MXS-1775 Introduce maxscale::MonitorInstance

- All monitors (but MariaDBMon for the time being) inherit
  from that.
- All common member variables moved to that class. Still
  manipulated in derived classes.

In subsequent commits common functionality will be moved to
that class.

server/modules/monitor/auroramon/auroramon.cc

@@ -26,19 +26,12 @@
 
 
 AuroraMonitor::AuroraMonitor(MXS_MONITOR* monitor)
-    : m_shutdown(false)
-    , m_thread(0)
-    , m_script(NULL)
-    , m_events(0)
-    , m_monitor(monitor)
-    , m_checked(false)
+    : maxscale::MonitorInstance(monitor)
 {
 }
 
 AuroraMonitor::~AuroraMonitor()
 {
-    ss_dassert(!m_thread);
-    ss_dassert(!m_script);
 }
 
 //static

server/modules/monitor/auroramon/auroramon.hh

@@ -20,7 +20,7 @@
  * @file auroramon.hh - The Aurora monitor
  */
 
-class AuroraMonitor : public MXS_MONITOR_INSTANCE
+class AuroraMonitor : public maxscale::MonitorInstance
 {
 public:
     AuroraMonitor(const AuroraMonitor&) = delete;
@@ -34,13 +34,6 @@ public:
     json_t* diagnostics_json() const;
 
 private:
-    bool         m_shutdown;      /**< True if the monitor is stopped */
-    THREAD       m_thread;        /**< Monitor thread */
-    char*        m_script;        /**< Launchable script */
-    uint64_t     m_events;        /**< Enabled monitor events */
-    MXS_MONITOR* m_monitor;       /**< Pointer to generic monitor structure */
-    bool         m_checked;       /**< Whether server access has been checked */
-
     AuroraMonitor(MXS_MONITOR* monitor);
     ~AuroraMonitor();