fixing race on grpc client start

[brettsam]

There's a possibility that _workerClient is null during initialization.

public async Task StartAsync(CancellationToken token)
{
    _workerClient = await _workerClientFactory.StartClientAsync(this, token);
}

Inside that call we send the StartStream() and start listening... so we can start to get messages sent to us before StartClientAsync() has returned -- leaving _workeClient null.

Change is to make this two distinct calls... one to create the client and another to start it so that it can never be null.