[tx] Allow setting the default partition for string temp IDs

For #184, I'm going to try implementing edn::Value::Text string temp IDs. By default, these will allocate in the :db.part/user partition.

This follows Datomic, but way down at the bottom of http://docs.datomic.com/transactions.html it shows a way to configure this "default partition". This ticket tracks letting this be configured. My initial implementation suggestion is to make this a datom in the store with a cached materialized view; might as well be consistent. Something like:

[:db/default :db.default/partition :db.part/user]

which reads "the DB defaults entity has default partition set to the user partition".