Modify readwritesplit to understand changes in server references

The readwritesplit now understands that the amount of servers can change
and some of the items in the list of server references aren't in use. This
allows dynamic changes to the number of servers used by readwritesplit.

server/modules/routing/readwritesplit/readwritesplit.h

@@ -281,7 +281,6 @@ struct router_client_session
     skygw_chk_t      rses_chk_top;
 #endif
     SPINLOCK         rses_lock;      /*< protects rses_deleted */
-    int              rses_versno;    /*< even = no active update, else odd. not used 4/14 */
     bool             rses_closed;    /*< true when closeSession is called */
     rses_property_t* rses_properties[RSES_PROP_TYPE_COUNT]; /*< Properties listed by their type */
     backend_ref_t*   rses_master_ref;