Schema version can be very wrong for Drupal 8 sites

Problem/Motivation

I have a Drupal 8 site that has a module, that module has an update hook from the Drupal 7 era (number 7100) still in the install file. This is perfectly valid 'Drupal' code, but provision incorrectly detects that the module has schema version 7100 whereas Drupal considers it to have version 8000. This means that I can't migrate the site to another platform, because Aegir refuses to allow me to do something stupid.

Proposed resolution

Provision needs to enforce a minimum schema version based on the new standard for Drupal modules:
https://www.drupal.org/node/2186315
I.e. the minimum version for Drupal 8 modules is not now 0, but 8000.

Remaining tasks

Evaluate and test the patch.

User interface changes

None.

API changes

None.

Data model changes

None.