doc: update TODO about missing invalidations in read-only mode

TODO

@@ -84,6 +84,8 @@
         keys (trans.tid & obj.{tid,oid}).
 
     Master
+    - Implement back-channel for invalidations in read-only mode,
+      so that clients of backup clusters are notified of new data.
     - Master node data redundancy (HIGH AVAILABILITY)
       Secondary master nodes should replicate primary master data (ie, primary
       master should inform them of such changes).

neo/lib/protocol.py

@@ -100,7 +100,6 @@ def ClusterStates():
     # invalidations and orders storage nodes to fetch them from upstream.
     # Because cells are synchronized independently, the DB is often
     # inconsistent.
-    # TODO: allow clients to connect for read-only operations
     BACKINGUP
     # Transient state, when the user decides to go back to RUNNING state.
     # The master stays in this state until the DB is consistent again.