Fix to bugs #665, and #664. Potentially to #649.

http://bugs.skysql.com/show_bug.cgi?id=665
http://bugs.skysql.com/show_bug.cgi?id=664
http://bugs.skysql.com/show_bug.cgi?id=649

dcb.c:dcb_final_free: (665):set dcb->session->client pointer to NULL so that it won't be read anymore and other threads won't try to close it.
	dcb_final_free:(664):don't free dcb->data, it is either freed in session_alloc if session creation fails or in session_free only.
session.c:if session creation fails, free dcb->data and remove links between client DCB and session.
mysql_backend.c:(665):gw_backend_close:check that session->client isn't NULL and that client DCB's state is still polling before calling dcb_close for it.
mysql_client.c:gw_mysql_do_authentication:if anything fails, and session_alloc won't be called, free dcb->data.
mysql_common.c:gw_send_authentication_to_backend:if session is already closing then return with error.

server/modules/include/mysql_client_server_protocol.h

@@ -111,9 +111,15 @@ typedef enum {
  *
  */
 typedef struct mysql_session {
+#if defined(SS_DEBUG)
+	skygw_chk_t	myses_chk_top;
+#endif
         uint8_t client_sha1[MYSQL_SCRAMBLE_LEN];        /*< SHA1(passowrd) */
         char user[MYSQL_USER_MAXLEN+1];                 /*< username       */
         char db[MYSQL_DATABASE_MAXLEN+1];               /*< database       */
+#if defined(SS_DEBUG)
+	skygw_chk_t	myses_chk_tail;
+#endif
 } MYSQL_session;