Import backups via descriptor

[cliffano]

Currently we only have support for importing a single backup package.
Similar to export backups, we need to introduce import backups, which iterates through the packages described in the export descriptor, with version (export timestamp) supplied by the user.

Example scenario: scheduled export backups run on 20180202, a user can then import backups from 20180202, which should import all packages described in the export descriptor.