Discard any client requests during view change

As of now, we don't process any client requests during view change (just not take them from the client's ZMQ queue while view change is in progress).

That may potentially lead to memory leaks. Most probably the client will get timeout for the request received during the view change, so it should not be a big issue to discard it.

Acceptance criteria:

• Process client queue during the view change (`serviceClientMsgs` in node.py), but discard all requests (send NACK) with a message "Discarding client request since View Change is in progress".
• Write tests