Improve transparency of the realm file.

[finnschiermer]

Idea to improve our ability to analyze realm files after a crash:

1. Add a signature to the top array, allowing us to identify top-arrays of earlier commits if present.
2. Add a link to the top array pointing to the top array of the previous commit, thus forming a chain of past commits. The link cannot be an ordinary ref, since it will not always be valid.
3. Add the "last reachable version" number to the top array, telling us how far back in time the chain of past commits are valid.

[BlueCobold]

This could help locating issues more easily and to restore old data in a corrupt file a lot. On the other hand, actually being ACID would help more.

[finnschiermer]

I can't disagree with that observation.