Add session state 'discard'

src/session/session_manager.h

@@ -53,6 +53,7 @@ struct session_stat
     uint64_t nr_sess_opening;
     uint64_t nr_sess_active;
     uint64_t nr_sess_closing;
+    uint64_t nr_sess_discard;
     uint64_t nr_sess_closed;
 
     uint64_t nr_new_sess_evicted;