cubicweb #1521112 database independent dump and restore [resolved]

As a developer, I need to be able to dump the database of an instance to a file, and restore that dump to another instance, which is possibly not running the same database backend.

As an instance manager, I need to be able to dump the db of my instance and restore it to another instance without having to ask special permission to the DBA of my IT department who are going to ask all kind of weird questions and make nasty remarks.

It would be nice if the restore part handled a completely empty database or a db-inited database, and was kind enough to empty the tables before restoring the contents.

done in3.12.4
load left0.000
closed by#d1c8b5b3531c adds support for a portable db import/export format (closes: #1521112)