Move to SessionExtension in vdm

[rgrp]

When vdm was created there was no SessionExtension so we use MapperExtension for doing revisioning. Now that SessionExtension? exists we should use it. We can also follow the existing SQLAlchemy recipe: <​http://www.sqlalchemy.org/docs/orm/examples.html?highlight=versioning#versioned-objects>

[rgrp]

Most of this change has been implemented in the changeset branch [1] but could be backported reasonably easily to main.

[1]: ​https://bitbucket.org/okfn/vdm/src/changeset/

[kindly]

This would be nice but it is not necessary. The current mapper implementation may be nice but not is battle tested..

[kindly]

Replying to kindly:

This would be nice but it is not necessary. The current mapper implementation may be nice but not is battle tested..

I meant the mapper extension "IS" battle tested.

[icmurray]

Re-assigned to @kindly.