Stub out the beginnings of the sdc-specific backend
Don't use stack for overlay lookups and responses

With the simple plugins that we have today, it's very easy to
effectively have a synchronous API. However, as we start the beginnings
of a varpd plugin that needs to have asynchronous behavior, then we will
not be able to rely on using stack-allocated queries as a given thread
that's processing requests may have to have more than one query open at
a time.