change the default Nexus session cookie name

The Nexus Session cookie name is currently JSESSIONID.

This is a default Java webapp default session cookie name.

When Nexus is running on a host where another java webapp is located, sometimes conflicts can occur with this cookie name where one webapp overwrites the cookie of the other.

To help avoid such conflicts, we should given the Nexus session cookie a non-default name. In case this presents unforeseen problems in some environments, make the cookie name configurable by a Nexus administrator.