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/server.c

@@ -39,7 +39,6 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <maxscale/service.h>
-#include <maxscale/monitor.h>
 #include <maxscale/session.h>
 #include <maxscale/server.h>
 #include <maxscale/spinlock.h>
@@ -51,6 +50,8 @@
 #include <maxscale/modules.h>
 #include <maxscale/gwdirs.h>
 
+#include "maxscale/monitor.h"
+
 /** The latin1 charset */
 #define SERVER_DEFAULT_CHARSET 0x08