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/test/testmodulecmd.c

@@ -21,6 +21,8 @@
 #include <maxscale/modulecmd.h>
 #include <maxscale/session.h>
 
+#include "../maxscale/monitor.h"
+
 #define TEST(a, b) do{if (!(a)){printf("%s:%d "b"\n", __FILE__, __LINE__);return 1;}}while(false)
 
 static bool ok = false;