Improvements to application notification & healthy store rebuilding.

[IMPROVED]  Notifying application more reliable.
[IMPROVED]  Set the correct cloudEnabled before clearing stores to avoid telling willLoadStore: the wrong status.
[IMPROVED]  Unset migrationStoreURL only on success so revertMigration reports the correct URL.
[IMPROVED]  Rebuild a healthy store only when the store is ubiquitous or finished the initial import.
[FIXED]     Don't revert when enabling cloud store fails, handle corruption instead.
[FIXED]     When disabling cloud store fails, load cloud store again instead of reverting migration (not migrating).
[IMPROVED]  DidLoad on the persistence queue now to be more reliably dispatched.