ManagedConsumer: queue size 1 behaves incorrectly

#### 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`