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/debugcli/debugcmd.c

@@ -53,7 +53,6 @@
 #include <errno.h>
 #include <maxscale/alloc.h>
 #include <maxscale/service.h>
-#include <maxscale/session.h>
 #include <maxscale/router.h>
 #include <maxscale/filter.h>
 #include <maxscale/modules.h>
@@ -78,6 +77,8 @@
 #include <maxscale/log_manager.h>
 #include <sys/syslog.h>
 
+#include "../../../core/maxscale/session.h"
+
 #define MAXARGS 12
 
 #define ARG_TYPE_NONE           0