Settle on ID conventions for pages/sessions/ancestors

[jsantell]

We generate string uuid's for each Page, and each Page is associated with a session ID (int), and for the most part, that mapping of page uuid -> session id is abstracted and contained within UserAgentHttpClient, except when we start adding the ancestor ID (a related session ID/int) to calls.

As a follow up to some discussions in #1560, maybe we can find a way to settle on naming (pageId vs id) or get rid of page IDs altogether and just use session IDs (if we can manage working with a tab before the session ID is created).