Discuss: Make hasMessageAvailable to interface Consumer.

Now the method `hasMessageAvailable` just is announced in ConsumerImpl and MultiConsumerImpl.
If we want to check is there message available, we should transfer Consumer to ConsumerImpl or MultiConsumerImpl,
then invoke hasMessageAvailable like below:
```
if (consumer instanceof ConsumerImpl) {
return ((ConsumerImpl) consumer).hasMessageAvailable();
} else if (consumer instanceof MultiTopicsConsumerImpl) {
return ((MultiTopicsConsumerImpl) consumer).hasMessageAvailable();
}
```

Maybe we can make method `hasMessageAvailable` to Consumer.

Start with the Consumer interface and the hasMessageAvailable implementations in ConsumerImpl and MultiTopicsConsumerImpl. Trace the existing call sites shown in the issue and check the related consumer tests; done means the availability check is exposed consistently through Consumer and the existing behavior remains covered.

Written by the indexing model from the issue text.