Implement oslo.versionedobjects indirection API

Right now, oslo.versionedobjects are used for object serialization when transferred by RPC. However versionedobjects, in their full utilization, allow specifying an indirection_api (VersionedObjectIndirectionAPI), which implements object backporting and making RPC calls transparently, by invoking object methods.

Backporting an object to a previous version can be used to run different code versions at the same time, facilitating upgrades without downtime and enabling rolling upgrades.

Making the RPC calls more transparent simplifies code development, by making the calls identical on server and client sides.

More information about versionedobjects can be found in the original proposal:
https://wiki.openstack.org/wiki/ObjectProposal

The goal of this blueprint is to implement the indirection_api taking the Nova implementation as a model.