Medium: cib: Include digests of the cib a diff was made from and verify it when applying
If the calculated and supplied digests do not match, then the diff is rejected
and a full copy requested. This ensures element ordering changes are preserved
and removes any possibility of the copies being out-of-sync.
This is an improvemnt on the previous method of verifying that the actual diff
matched the supplied one in that it only checked parts of the cib modified by
the update.

Mercurial revision: 61a6d48fb3181ba5c454bfce70fd46c1d10b573e