Fix CloudKit data loss on delete-then-reinsert

[jsutula]

Fixes #418

Root cause

Reinsertion of a record after a delete does not produce a new pending .saveRecord change entry due to missing "undelete" state detection in afterInsert trigger. No new .saveRecord means that only the .deleteRecord is propagated.

Solution

Two parts:

1. Prerequisite change necessary for writing accurate tests: Update MockSyncEngineState to deduplicate changes across types like the real CKSyncEngine.State

From CKSyncEngine.State.add(pendingRecordZoneChanges:) documentation:

Note
The order in which you apply record zone changes is important.
For example:

• If you add .saveRecord(recordA) then .deleteRecord(recordA), the sync engine
  discards the save and sends only the delete change.
• If you add .deleteRecord(recordA) then .saveRecord(recordA), the sync engine
  discards the delete and sends only the save change.

MockSyncEngineState.add(pendingRecordZoneChanges:) deduplicates fully identical changes (type+ID) by virtue of the underlying OrderedSet, but does not deduplicate when the type (save vs delete) is different, as is described above with CKSyncEngine.State. MockSyncEngineState.add(pendingDatabaseChanges:) has a similar issue. The fix: update both to deduplicate based on ID alone which will allow for the cross-type deduplication described above, and add tests.

2. Primary fix: Detect "undelete" on reinsertion in order to queue new .saveRecord change

Update the afterInsert trigger to conditionally reset _isDeleted to false if it's been set to true (as it will be in a delete-then-reinsert scenario), and update userModificationTime to the current time (as would occur in a normal insert or update). Hook up a new afterUndeleteTrigger that listens for this specific change and queues a .saveRecord change via syncEngine.$didUpdate (just as would occur on a normal insert or update).

Testing

In addition to new unit tests, performed a manual test with 9ea5bdd applied to verify that the data loss issue no longer occurs. Result (compare with before):

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