Properly override ApplicationSession c-tor, supports Crossbar.io.

The way we overrode the ApplicationSession constructor in our
ServerProtocol class was working for us when we invoked it from
server.py, but it was not allowing Crossbar.io to construct the
class.

Change-Id: I7061cfe131c2a376f366804281ffac4897ae2a85
Merge topic 'support-future-use-of-crossbar-io' into master

12f6589c Properly override ApplicationSession c-tor, supports Crossbar.io.