Decide whether ids need to be globally unique or not

[jrudolph]

Since all entities share the same entity log, right now entity ids need to be unique across all entity types. Either the queries could be improved not to make this assumption (add another WHERE clause everywhere) or this assumption needs to be documented.

[hseeberger]

Hmm ... so change UNIQUE (entity_id, version) to UNIQUE (entity_id, type_name, version)?

[jrudolph]

Hmm ... so change UNIQUE (entity_id, version) to UNIQUE (entity_id, type_name, version)?

Yes that, and also add an extra constraint on the type to all queries.

[jrudolph]

I actually already ran into this problem because you can just load the entity of a certain id with the wrong entity type and it will happily recover that entity (which somewhat likely fails during deserializing the wrong event types but might not depending on lucky circumstances).

[jrudolph]

Done in #38.