Add custom ssl context support to druid AsyncHttp requests.

[Chandrasekar-Rajasekar]

The Async Http Request should accept custom SSL context that contains the certificate and private key for authentication. AbstractBinderFactory needs a new method getSSLContext() and the SSL context returned by this method is used to establish the connection with the druid.

protected SslContext getSSLContext() {
        return null;
    }

AbstractBinderFactory Implementation has to override this method to provide their own custom SSL context.