Restore compatibility with Jetty 8 when SSL is being used

ServerConnector is in different packages in Jetty 8 and Jetty 9 which
was leading to a NoClassDefFounderError when SSL was used with
Jetty 8.

This commit updates SslServerConnectorFactory to return an
AbstractConnector, a super class of ServerConnector, that is in the
same package in both Jetty 8 and Jetty 9. This class does not provide
a setPort method so the setting of the port has been pushed down into
the SslServerConnectorFactory implementation.

SSL samples for both Jetty 8 and Jetty 9 have been added to verify
SSL with both supported versions of Jetty.

Closes gh-1925