Fix: Handle peer KEEPALIVE messages in chaincode message handler

[dzikowski]

When Node.js chaincode receives keepalive message from the peer, it treats it as unknown message - it is not handled by Node.js chaincode shim. It produces the unknown message error and exits the chaincode process.

In such case the following logs are produced:

<date> debug [c-api:lib/handler.js] Received chat message from peer: , state: ready, type: 18
<date> error [c-api:lib/handler.js] Received unknown message from the peer. Exiting.

It turns out the Go shim handles keepalive message (source):

if msg.Type == peer.ChaincodeMessage_KEEPALIVE {
    h.serialSendAsync(msg, errc)
    return nil
}

This PR adds the same behavior to the node shim:

            if (msg.type === MSG_TYPE.KEEPALIVE) {
                stream.write(msgpb);
                return;
            }

[bestbeforetoday]

Thank you very much for this contribution and for including tests.