Add an API for retrieving the events stored for a particular user
Add a test to see if the basic functionality for retrieving user events is working
Thinking about how this needs to work. We need to know the user ID when recording an event, not just the session ID (this used to make sense for the SQL query generator).

Increasingly, the 'StartSession' call is looking less useful...
Add a way to associate an event with a user
In order to enumerate the events we associated with a user, we will need to enumerate across the set of values beginning with a particular key
Add a test for the new 'retrieve items with prefix' enumerator
Implement the prefix enumeration for the memory store
The per-user event enumeration will be easier to implement if we store the event twice instead of just the event ID (this will slow down deletion later, but it does save us needing to enumerate twice for now)
Use the new API call to make it possible to retrieve the events for a user.
Add a test to check that the event contents look right when we retrieve them this way