Switch to new vdm changeset model

[rgrp]

Have developed a new "changeset" based model for revisioning in vdm. This has several advantages:

• Much simpler
• Cleaner separation of continuity from changesets
  • Supports certain operations that are impossible now (e.g. deleting all changes to a particular object irrespective of whether other objects were changed in same revisions).
• Easier support for pending state and similar behaviour
• Almost identical API

Implementation

• The main challenge with this change is schema and data migration

Migration

Every revisioned object has a revision_id and revision attribute.

Approximate algorithm:

Revision -> Changeset

for revtype in [PackageRevision, ...]:
    for pkgrev in package_revision:
        changeset = lookupchangeset(package_revision)
        ChangeObject(cset, (table, id), dictize(pkgrev))

Question:

• does pkg include tags attributes or not? or we have to dictize, pkgrev, pkg2tagrev, and tag. Probably the latter.