add backup database schema to evolove; break server database

Add database schema versions to the Obnam client and make the client handle any version.

Rename the "sha256" column in the server to "label". This is a breaking change: you'll have to re-do all your backups, sorry. (I hope to get the server also into shape to avoid such breaking changes in the future, but it was too much for this iteration.)

Sponsored-by: author

Closes #192 (closed), #194 (closed)

I will merge this on Tuesday, unless there are ongoing discussions.