BTCloudKitSync

Replace deprecated CKSubscription initializer with CKRecordZoneSubscription

Adding in a few missing completion handlers. If the code experienced an error during some operations, there was a case where the app would get stuck in sync.

Added in notifications for sync progress and implemented something basic in the demo app.

Added a shouldAllowSync -> Bool to the BTCloudKitSyncDatabase protocol to let consuming apps control when sync should be allowed.

Changed the way that sync is kicked off automatically at startup so that it doesn't just send the local database's message (which could mess up the consuming app's UI ... especially since this isn't on the main thread).