Implement `db.fn/cas` for compare-and-set semantics

[rnewman]

https://docs.datomic.com/on-prem/transactions.html#dbfn-cas

[ncalexan]

I thought about this at one point, and concluded that the best way to implement this was a post checker that verifies the CAS conditions after doing the searches against existing data, but before committing the transaction.

[rnewman]

Aye, that's feasible. But it does mean introducing some error state in InProgress — if the transaction aborts then we need to get back up the stack and block the InProgress, not just the SQL transaction.