public static <I, O> Builder<I, O> builder() {
        return new Builder<I, O>(new Func1<Builder<I, O>, LoadBalancingHttpClient<I, O>>() {
            @Override
            public LoadBalancingHttpClient<I, O> call(Builder<I, O> builder) {
                return new LoadBalancingHttpClient<I, O>(builder);
            }
        });
    }
Strategy for calculating the backoff based on the number of reties.  Input is the number
of retries and output is the backoff amount in milliseconds.
The default implementation is non random exponential backoff with time interval configurable
via the property BackoffInterval (default 1000 msec)

@param backoffStrategy
ribbon-transport/src/main/java/com/netflix/ribbon/transport/netty/http/LoadBalancingHttpClient.java