This patch does several things, all in one commit because of a schema update:
* Uses _id instead of guid when referring to reading list items, allowing the guid column to be null.
* Reworks schema upgrading.
* Completely revises the reading list schema itself.
* Fixes the tests.
* Cleans up how we do deletion: if an item hasn't yet been synced, it's simply deleted immediately. We can do this because the server allocates GUIDs.
* Adds columns to manage sync-related metadata.
Centralizing reading list access logic will make Bug 1130461 much easier. This bug is the first part of that.
We follow the same pattern as for URLMetadata, TabsAccessor, and Searches; BrowserDB hands over a single class that's specialized to handle the Reading List.
