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.

include/maxscale/monitor.hh

@@ -14,10 +14,31 @@
 
 #include <maxscale/cppdefs.hh>
 #include <maxscale/monitor.h>
+#include <maxscale/thread.h>
 
 namespace maxscale
 {
 
+class MonitorInstance : public  MXS_MONITOR_INSTANCE
+{
+public:
+    MonitorInstance(const MonitorInstance&) = delete;
+    MonitorInstance& operator = (const MonitorInstance&) = delete;
+
+    virtual ~MonitorInstance();
+
+protected:
+    MonitorInstance(MXS_MONITOR* pMonitor);
+
+    int          m_status;   /**< The current status of the monitor. */
+    THREAD       m_thread;   /**< The thread handle of the monitoring thread. */
+    MXS_MONITOR* m_monitor;  /**< The generic monitor structure. */
+    int32_t      m_shutdown; /**< Non-zero if the monitor should shut down. */
+    char*        m_script;   /**< Launchable script. */
+    uint64_t     m_events;   /**< Enabled monitor events. */
+    bool         m_checked;  /**< Whether server access has been checked. */
+};
+
 /**
  * The purpose of the template MonitorApi is to provide an implementation
  * of the monitor C-API. The template is instantiated with a class that