[WIP] Start of a potential transition to zope.interfaces.

zope.interfaces provides quite a few benefits over ABCs most notably a wealth of metadata that allows for programmatic verification of classes and instances that claim to implement or provide a given interface (#1024).

This PR demonstrates how to convert a single ABC->Interface in a way that is compatible with our usages of ABCs and demonstrates the addition of a unittest for verifying that multibackend provides that interface.