poll.c: added maxscale thread id to log

session.c: Replaced free(session->router_sesision) with call to freeSession callback
users.c: Removed reference to uninitialized variable.
router.h: Added freeSession callback to function block.
mysql_backend.c: try to ensure that client dcb is still listening in epoll_wait when writing reply to it.
mysql_common.c: assert debug build is mysql_protocol_init is called with dcb == NULL
readconnroute.c, readwritesplit.c, debugcli.c and testroute.c : Added freeSession to function block and an inmplementation of it.

server/core/users.c

@@ -114,7 +114,6 @@ int	del;
 
 	atomic_add(&users->stats.n_deletes, 1);
         if (users->stats.n_entries == 1) {
-            atomic_add(&users->stats.n_entries, del * -1);
             return 0;
         }
 	del = hashtable_delete(users->data, user);