[tx] Allow [:db/tx :db/txInstant INSTANT] in transaction input

[ncalexan]

Datomic (and the Clojure implementation of Mentat) allow to specify the :db/txInstant that the transaction will be applied with by including a datom like

[[:db/add :db/tx :db/txInstant INSTANT]]

(Note that the :db/tx is a Mentat addition, although I think Datomic added an equivalent thing recently.) This requires INSTANT to be strictly after the last transaction timestamp and strictly before the current transactor timestamp.

Adding this allows to import data with meaningful timestamps, for example.

[rnewman]

I don't believe it's necessary that the timestamp be later than the last transact: multi-master replication demands the opposite. But it's a reasonable restriction to start with.

[ncalexan]

Let's make this ticket track also adding arbitrary assertions with the :db/tx entity, that is, handling things like

[[:db/add :db/tx ATTRIBUTE VALUE]]

[rnewman]

3bf7459