[SCHEMA] s/account_id/namespace_id for contacts,events,calendars.

This makes contacts and events consistent with the rest of our objects, wihich
we partition by namespace rather than contact.
[WIP] Update messages API to filter directly by Message.namespace_id.

We formerly had to join messages to threads in order to filter by namespace_id,
which had grim applications for performance. Now that messages have a
namespace_id themselves, don't do this.