Git commits in Moodle 2.9.3 by Pau Ferrer Ocaña

commit 00a51457f4fe4195c4a3b2ffaacc0e9fc5b609e9
Author: Pau Ferrer Ocaña <crazyserver@gmail.com>
Date:   Wed, 23 Sep 2015 23:02:39 +0800
Tag:    v2.9.3

MDL-50805 Messages: Add index to message_read to speed up message crons