Get page view, session, and user IDs from mini core.

[sholladay]

Before the mini core can be brought online, Sitecues must be made aware of how it persists page view IDs, session IDs, and user IDs, as mentioned in sitecues/sitecues-core#1.

The goal is to ensure that these IDs are consistent across metrics sent by all libraries. Thus, the top priority is for API flexibility and stability, to avoid dependency hell.

A reference implementation can be found in sitecues-mobile for all of them:

• page view
• session
• user