Unbreak test suite.

Updates CoreDataHelpers to call ATHNotify() on root context save.
Updates methods to use AccountService instead of WPAccount where needed.
Removes some obsolete code involving background contexts.

Fixes #1613 

With this all but two of our tests are passing for me, one for Jetpack and one data concurrency. <del>I think we can look at those as individual issues.</del> Or @astralbodies will just be awesome and fix them. :)