* Resurrect symmetric vkeys when possible.
AbstractVKeyConverter had never been updated to generate symmetric VKeys for
simple (i.e. non-composite) VKey types. Update it to default to generating a
VKey with a simple Ofy key.
With this change, composite VKeys must specify the "compositeKey = true" field
in the With*VKey annotations. Doing so creates an asymmetric (SQL only) VKey
that can triggers higher-level logic to generate a corrected VKey containing
the composite Key.
