Automatic content corruption resolution.

[IMPROVED]  Content corruption is handled much better now:
                - We keep track of whether the store is corrupt or only the content.
                - We record store and content as corrupt when the store fails to load too.
                - The default behavior can now often automatically recover corruption
                  when the user opens the app on another device.
[FIXED]     Situation where a double-fix of the cloud content might erase it all.
[ADDED]     Rebuild might fail when the cloud store is not viable or gone, possibility of falling back to rebuilding from the local store.
[FIXED]     Watching for cloud content deletion: deadlocks and bad test.
[IMPROVED]  Documentation and a bunch of bugs and side-effects throughout.
[UPDATED]   The example now shows a spinner telling the user to open the app on another device or giving him an option to override and rebuild anyway.