Implement partial backups

[ecton]

After #119, we now have basic backups implemented with a way to add additional backup locations. We should be able to store one additional piece of data, the current transaction id, and be able to update the backup process to operate in two modes: full and partial.

In a partial backup, we scan the transaction log looking for all keys and documents that have changed, and only backing up the items that have changed.

This will require the backup storage to support deleting individual stored keys.