SQL modelling

[mpardalos]

I have an implementation of a lot of the transaction features in SQL, however I'm still missing some things that might be useful, most notably a notion of queries (every event is now considered as it's own "query") and serializable transactions.

I'm leaving this draft PR here as a place to make notes that might be of interest. I'm marking it as draft since I haven't found any interesting executions yet and I also like to add some things (like the weakening we talked about). If you think it should be merged in its current state I'd be happy to unmark it.