Pass session ID in cross-domain tracking in addition the user ID

[matus-tomlein]

The cross-domain tracking feature of the tracker attaches a _sp parameter to the querystring of outbound links on the page with information about the current domain user ID and timestamp. This then populates the refr_domain_user_id column when the links are clicked and events tracked from the destination page.

In addition to the user ID and timestamp, it would be useful to also pass the domain session ID in the parameter to give more information about the source user and session.