Migrate last transaction log when upgrading from 1.9 to 2.0.

This allows incremental backups and cluster joins to function after store upgrades,
because they can now always figure out what their last transaction id, and its
checksum, is.
Merge pull request #2626 from chrisvest/2.0-migrate-last-log

Migrate last transaction log when upgrading from 1.9 to 2.0.