MXS-1775 Inherit MariaDBMonitor from mxs::MonitorInstance

Start/stop now provided by MonitorInstance. The thread main
function is now virtual and overriden by MariaDBMonitor. Some
additional refactoring is necessary in order to be able to allow
MonitorInstance to handle the main loop.

include/maxscale/monitor.hh

@@ -103,6 +103,16 @@ protected:
     const std::string& script() const { return m_script; }
     uint64_t           events() const { return m_events; }
 
+    /**
+     * @brief Should the monitor shut down?
+     *
+     * @return True, if the monitor should shut down, false otherwise.
+     */
+    bool should_shutdown() const
+    {
+        return atomic_load_int32(&m_shutdown) != 0;
+    }
+
     /**
      * @brief Configure the monitor.
      *
@@ -165,6 +175,11 @@ protected:
      */
     virtual void tick();
 
+    /**
+     * TODO: Temporarily virtual so that MariaDBMonitor can override.
+     */
+    virtual void main();
+
     MXS_MONITOR*          m_monitor;  /**< The generic monitor structure. */
     MXS_MONITORED_SERVER* m_master;   /**< Master server */
 
@@ -177,8 +192,6 @@ private:
     uint64_t    m_events;    /**< Enabled monitor events. */
     Semaphore   m_semaphore; /**< Semaphore for synchronizing with monitor thread. */
 
-    void main();
-
     static void main(void* pArg);
 };