Revive the root_kommit.

This will be the first commit on the history graph.
Kommit digests must be unique.
More work to revive the root kommit.
Restore the old IndexWriter.
The register/unregister and pin/unpin methods are moving to the Distribution class.
Remove test cases for reverting.

Merging and Reverting will not be supported (for now).
Revive the RegistrationChange schema class.
Restoring more logic.