[DISCUSS] Change the default IO threads and listener threads of Java Client

[PengHui Li]

Hi all,

I noticed the Java Client (I haven't checked other clients) uses 1 IO
thread and 1 listener
thread by default. It will require users to update the thread configuration
if they have
multiple cores and desired high throughput.

Here is the example that we change to 16 IO threads in
openmessaging benchmark
https://github.com/openmessaging/benchmark/blob/master/driver-pulsar/pulsar.yaml#L22

We can apply the configuration of the threads based on the CPU cores. So
that for the
most common cases, users don't need to touch the thread configuration.

```
private int numIoThreads = Runtime.getRuntime().availableProcessors();
private int numListenerThreads = Runtime.getRuntime().availableProcessors();
```

WDYT?

Thanks,
Penghui