Add VERSION column, implements #778

This commit adds a read-only task VERSION column which is incremented upon any update or a task (including sync-adapter updates).

The column serves two purposes:

* safe transactions by asserting a specific version before updating a task
* quick detection if a specific task has been modified

The former could be used by sync-adapters to ensure a task has not been updated while being synced.
Notifications will make use of the latter to avoid unnecessary updates of task notifications for unchanged tasks.