ManagedConsumer: queue size 1 behaves incorrectly

[awilliams]

Expected behavior

When using a managed consumer with QueueSize == 1, eg

c := pulsar.ManagedConsumerConfig{
	QueueSize:           1,
}

Then it's expected that c.ReceiveAsync receive messages.

Actual behavior

The managed consumer incorrectly sets the highwater to 0, which incorrectly performs flow control.

System configuration

v0.0.1