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/modules/include/readwritesplit.h

@@ -224,6 +224,7 @@ typedef struct backend_ref_st {
         bref_state_t    bref_state;
         int             bref_num_result_wait;
         sescmd_cursor_t bref_sescmd_cur;
+	GWBUF*          bref_pending_cmd; /*< For stmt which can't be routed due active sescmd execution */
 #if defined(SS_DEBUG)
         skygw_chk_t     bref_chk_tail;
 #endif