I found an `_asyncSupported` private member variable in Jetty's `Holder<T>` class, a base class for the `ServletHolder` class that we use to programmatically register servlets with Jetty.  In Holder.java, I see:

```java
protected Holder(Source source)
    {
        _source=source;
        switch(_source)
        {
            case JAVAX_API:
            case DESCRIPTOR:
            case ANNOTATION:
                _asyncSupported=false;
                break;
            default:
                _asyncSupported=true;
        }
    }
```

In the trapperkeeper-webserver-jetty9 service, we construct a `ServletHolder` instance using the form of a constructor that doesn't specify a `Holder.Source`.  In ServletHolder.java, I see:

```java
 public ServletHolder(Servlet servlet)
    {
        this(Source.EMBEDDED);
        setServlet(servlet);
    }
```

Since `Source.EMBEDDED` would translate to _asyncSupported = true, I think we're getting the async behavior by default for the servlet when we programmatically register it.

I'm good with the change you made since it doesn't appear to be used as a servlet init param anyway and I don't see that we need to make any additional changes if we're already getting async behavior.  Special-casing the `ServletHolder` initialization for the `ProxyServlet` to be more explicit seems like it could be problematic.