This implements a backend layer and database storage for tracking what
wikis a user has unread notifications on. It is not yet exposed via any
API.
Whenever the notification counts on the local wiki are reset, a deferred
update is queued to also update the central database table.
Change-Id: Id1498bdeb5811d6848dc66781ffca03e726eab90
* Get rid of EchoBackend by separating responsibilities into smaller objects
* Move main fetchNotification logic from API to a more appropriate place
* Add more unit testing coverage
Change-Id: I42f4d7566543332588431c21c220c0d64d026b70
