Reloading Inconsistent Entities

[jleandroperez]

Details:

In this PR we're adding an error handling mechanism, capable of detecting Ghost(s) in inconsistent states, and triggering a full entity re-download.

cc @bummytime
cc @dmsnell

Closes #584

Testing:

1. Check out Simplenote iOS
2. Map Simperium's dependency to commit bbcbc91 (this branch, latest!)
3. Comment out DiffMatchPach.m L1510 thru L1528
4. Clean + Rebuild
5. Log into your account
6. Add a new note
7. Insert the following emojis: ☺️🖖🏿
8. Insert the following emoji in between: 😃

• Verify Simplenote does not crash
• Verify that (after a second) you end up with ☺️😃(null)🖖🏿 onscreen

Note:

• The purpose of this fix is to prevent the app from entering a crashloop, caused by NSJSONSerialization while attempting to serialize a broken string, and recover by performing a full entity re-download
• The actual underlying bug has been already fixed via DiffMatchPatch: Fixing Breaking Scenario #583 and Bugfix: DiffMatchPatch Surrogate Pairs Support #582

[bummytime]

@jleandroperez Ok I tested this pretty well.

✅ code
✅ unit tests

I also tested this branch against the release/4.14.2 hotfix branch for Simplenote:

1. Fetched, built, installed tags/4.13 and ran + added the spock note (crashed!)
2. Fetched, built, installed release/4.14.2 and ran it (crashes on startup)
3. Fetched this branch against release/4.14.2 and ran it (no crash):

!!

[jleandroperez]

Thank you for testing this one Bummy!!