Prevent multiple closes

`AsyncHttpClient.close()` and `NettyAsyncHttpProvider.close()` (and probably other providers) should prevent multiple invocations by using CAS like this:

``` java
if (isClosed.compareAndSet(false, true) {
    // release resources
}
```

p.s. also `AsyncHttpClient.closeAsynchronously()` can just delegate to `AsyncHttpClient.this.close()` from `Runnable`.
