Allow for two-way ssl(SSLEngine.setNeedClientAuth())

[BroHammie]

CustomSSLWebSocketServerFactory doesn't allow for full customization of the SSLEngine. Addition of a SSLEngineWebSocketServerFactory that takes SSLEngine as a parameter allows for configuring much more than was exposed in the CustomSSLWebSocketServerFactory.

See: https://docs.oracle.com/javase/7/docs/api/javax/net/ssl/SSLEngine.html

[marci4]

This issue was implemented with #839 !