Remove desync avoidance.

[REMOVED]   Desync avoidance strategies were unreliable and only worked while all devices are online.
More robustness.

[IMPROVED]  More robust handling of PSC locking and operation queues to avoid locking up under certain peculiar conditions (eg. deleting the container from another device or from the local device).
[IMPROVED]  Delete the stale store file when the store content is deleted to prevent corrupt cloud content from being created.
[IMPROVED]  Perform store deletion asynchronously.
[IMPROVED]  Notify the application immediately whenever the cloud store becomes unavailable so it can unset its MOCs and re-initialize its other persistence users (eg. fetchedResultsController).
A few doc updates.

[UPDATED]   A few more details about default implementations.