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.c

@@ -34,7 +34,6 @@
 #include <time.h>
 #include <maxscale/alloc.h>
 #include <maxscale/service.h>
-#include <maxscale/session.h>
 #include <maxscale/server.h>
 #include <maxscale/router.h>
 #include <maxscale/modules.h>
@@ -53,6 +52,8 @@
 #include <maxscale/secrets.h>
 #include <maxscale/users.h>
 
+#include "../../../core/maxscale/session.h"
+
 extern char *create_hex_sha1_sha1_passwd(char *passwd);
 
 static int maxinfo_statistics(INFO_INSTANCE *, INFO_SESSION *, GWBUF *);