How to do KVS

[viktorklang]

Having support for Key-Value style state management would be interesting,
it could either be implemented on top of CRUD #50 by the Entity being a Map.

This also impacts the user-facing API as they would not have to deal with anything but the inbound commands (not events), by adding/modifying/removing the values in the Map.

[kwehden]

We should consider doing this on Postgres, to demonstrate that we work with other persistence stores outside of Cassandra. This I think is a perceived bias that we've seen in some feedback.

[viktorklang]

@kwehden Agreed—I'm a big fan of Postgres as an RDBMS

[andyczerwonka]

@kwehden & @viktorklang using native hstore and/or JSONB within Postgres may make sense for this use case