Added spinlock ses_lock to struct SESSION to ensure that for each session closeSession is called only once.

closeSession is called from mysql_backend.c:gw_read_backend_event, and from dcb.c:dcb_close.

This is part of Bug #163.

server/core/session.c

@@ -59,6 +59,7 @@ SESSION 	*session;
 
 	if ((session = (SESSION *)malloc(sizeof(SESSION))) == NULL)
 		return NULL;
+        spinlock_init(&session->ses_lock);
 	session->service = service;
 	session->client = client;
 	memset(&session->stats, 0, sizeof(SESSION_STATS));