Fold over DBs on disk rather than load into memory
Fold over DBs on disk rather than load into memory

This uses the new mem3_shards:fold API to walk the shards from the
on-disk representation.

BugzID: 13504

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb