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/core/monitor.cc

@@ -14,7 +14,7 @@
 /**
  * @file monitor.c  - The monitor module management routines
  */
-#include <maxscale/monitor.h>
+#include <maxscale/monitor.hh>
 
 #include <fcntl.h>
 #include <stdio.h>
@@ -2502,3 +2502,24 @@ int mon_config_get_servers(const MXS_CONFIG_PARAMETER* params, const char* key,
     }
     return found;
 }
+
+namespace maxscale
+{
+
+MonitorInstance::MonitorInstance(MXS_MONITOR* pMonitor)
+    : m_status(0)
+    , m_thread(0)
+    , m_monitor(pMonitor)
+    , m_shutdown(0)
+    , m_script(NULL)
+    , m_events(0)
+{
+}
+
+MonitorInstance::~MonitorInstance()
+{
+    ss_dassert(!m_thread);
+    ss_dassert(!m_script);
+}
+
+}