Like the section that describe it [0], pluggable storage engine,
the purpose of nstore-engine is to make it possible to switch
between SRFI-167: okvs implementation while re-using the
same nstore code.

[0] https://srfi.schemers.org/srfi-168/srfi-168.html#pluggable-storage-engine

That is what a pluggable backend is.

I was under the impression that this was similar to comparators kind of
object that configures a given library using a record that bundle a
few procedures.

Now, the problem with `engine` object, is that it prescribes a particular
implementation of nstore that rely on okvs. Do we want that?