MXS-1889 Handle master used as slave

Up until 2.1.12, if it in the configuration file said
'router_options=slave', the master was used if there were no
slaves at session creation time.

That broke in 2.1.13 as a side-effect of MXS-1516 that checks
at routing time whether the server initially selected as master
still is the master.

Now the required server status is stored separately for each
session, so that if the master was chosen, even though we have
'router_options=slave', we can turn on the SERVER_MASTER bit.

That allows us to handle the case correctly in connection_is_valid().

server/modules/routing/readconnroute/readconnection.h

@@ -49,6 +49,7 @@ typedef struct router_client_session
     SERVER_REF *backend; /*< Backend used by the client session */
     DCB *backend_dcb; /*< DCB Connection to the backend      */
     DCB *client_dcb; /**< Client DCB */
+    unsigned int bitvalue; /*< Session specific required value of server->status */
     struct router_client_session *next;
 #if defined(SS_DEBUG)
     skygw_chk_t rses_chk_tail;