Divide monitor.h to internal and interface headers similar to filter.h

Definitions and function used by core are in
server/core/maxscale/monitor.h
Definitions and function used by plugins are in
include/maxscale/monitor.h

server/core/config_runtime.c

@@ -20,6 +20,7 @@
 
 #include "maxscale/config.h"
 #include "maxscale/service.h"
+#include "maxscale/monitor.h"
 
 static SPINLOCK crt_lock = SPINLOCK_INIT;