Fix to #478, http://bugs.skysql.com/show_bug.cgi?id=478

Execution of session commands failed because session commands and normal sql statements were executed in a wrong order if backend was executing previous session command while new sql stmt was routed to that backend. There was a window where ordering went wrong.
It is possible that one normal sql stmt arrives while previous sescmds are still being executed.

Introduced a new member in backend_ref_t structure, bref_pending_cmd where new sql stmt pointer is stored in that case. When sescmds are executed completely, that command is automatically executed next.

server/core/dcb.c

@@ -485,7 +485,6 @@ bool    succp = false;
                 /*<
                  * Close file descriptor and move to clean-up phase.
                  */
-		ss_dassert(excluded != dcb);
                 rc = close(dcb->fd);
 
                 if (rc < 0) {