we should have local-only keys

[phritz]

this has come up several times and seems totally natural to have a keyspace of some sort that is not synced

in other contexts eg https://github.com/rocicorp/replicache/issues/79 we have discussed having a keyspace where client is the source of truth that is synced to the server. maybe when considering how local-only should work consider the question in light of a full set of keyspaces (server truth syncd, client truth syncd, client unsyncd).