Separate snapshots from current state

[maparent]

Currently in a single table with an "obsolete" column for snapshots.
There should be multiple history storage cases, defined as part of the projection schema:

1. No history (current state only)
2. Separate snapshots: Have both a current state and a snapshot history table (in another database schema?)
3. Integrated snapshot (as now: a single table with an obsolete column)
4. Full history: integrated snapshot, record every change, allows simplest time travel possible.

In cases 1 and 2, this allows foreign keys.
Case 4 allows can be used for easy time travel on (eg) name bindings.