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/protocol/mysql_common.c

@@ -1642,6 +1642,8 @@ void protocol_archive_srv_command(
         server_command_t*  h1;
         int                len = 0;
         
+	CHK_PROTOCOL(p);
+	
         spinlock_acquire(&p->protocol_lock);
         
         if (p->protocol_state != MYSQL_PROTOCOL_ACTIVE)
@@ -1692,6 +1694,7 @@ void protocol_archive_srv_command(
         
 retblock:
         spinlock_release(&p->protocol_lock);
+	CHK_PROTOCOL(p);
 }