Make and restore snapshots our own way

[jchappelow]

We went the route of pg_dump and psql for convenience, but we should now look at creating and restoring snapshots in kwild's way.
Since our app controls access to the DB (we can pause or break out of normal sync / execution), we don't need pg_dump to create a snapshot transaction and tax the system with locking all the things globally.
The myriad of exclusions, inclusions, and rewrites has become challenging and error prone. Then there's the generally unpleasant need to use binaries installed on the system (see dependency checks and difficulty satisfying them).

I think we can get this done more easily now, particularly since all known user namespaces are tracked in info namespace, and we can centralize a registry of internal kwild schemas.