By introducing the "shared rocksdb instance" concept to the backend, we
can remove the "piggybacking" mode , thus reducing the complexity of
database initialisation and opening the possibility of extending how
write batching works across kvt/aristo.

The change makes explicit the hidden shared state that was previously
hiding in closures and provides the first step towards simplifying the
"commit/persist" interface of coredb, preparing it for optimizations to
reduce the "layering tax" that `forked-layers` introduced.