Divide session.h to internal and interface headers

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

server/modules/routing/maxinfo/maxinfo_exec.c

@@ -31,7 +31,6 @@
 #include <string.h>
 #include <maxscale/alloc.h>
 #include <maxscale/service.h>
-#include <maxscale/session.h>
 #include <maxscale/router.h>
 #include <maxscale/modules.h>
 #include <maxscale/monitor.h>
@@ -47,6 +46,8 @@
 #include <maxscale/resultset.h>
 #include <maxscale/config.h>
 
+#include "../../../core/maxscale/session.h"
+
 static void exec_show(DCB *dcb, MAXINFO_TREE *tree);
 static void exec_select(DCB *dcb, MAXINFO_TREE *tree);
 static void exec_show_variables(DCB *dcb, MAXINFO_TREE *filter);