Change user_id to be set on a per-event basis

[alexanderdean]

If we share a Tracker object between multiple different requests, should we be passing user_id in on a per-event basis, not setting it at the Tracker level...

/cc Fred Blundun (@fblundun) Yali (@yalisassoon)

[yalisassoon]

Agree we should be passing it in on a per-event basis. This would work for Custom Made, be consistent with the approach to recording timestamps in the Python tracker and is consistent with our current approach to unstructured context.

[alexanderdean]

Let's architect this by:

• Moving all the trackXXX events into a new Subject class
• The Subject can have its user_id set (or re-set)

[fblundun]

Rescheduling for 0.4.0...