[tx] Implement :db/tx, :db/txInstant in transactor

[ncalexan]

I just didn't get to tracking transaction IDs at all in #170. This might require #184, depending on how it's implemented, but there are simpler ways to achieve it.

Each transaction gets an extra assertion of the form:

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

where :db/tx is the next sequential transaction ID (in the :db.part/tx partition), and INSTANT is the transactors local time. (We'll ignore clock drift for now -- at our own peril.)

[rnewman]

Is this done, @ncalexan?

[ncalexan]

Is this done, @ncalexan?

Yep. Worth linking to #225, which tracks allowing the consumer to use :db/tx and :db/txInstant in the transaction.

[rnewman]

OK. Closing this one!