Nested upserts to the same entity by identity fails

Opening a separate issue based on comment at https://github.com/tonsky/datascript/issues/76#issuecomment-269110873

In order the following fails with Cannot add #datascript/Datom [1 :name "Ivan" true] because of unique constraint, but succeeds if you swap the order of keys in the map.

(deftest test-ref-upsert
  (let [db (d/empty-db {:name {:db/unique :db.unique/identity}
                        :ref {:db/valueType :db.type/ref}})]
    (are [tx res] (= res (tdc/all-datoms (d/db-with db tx)))
      [(array-map :ref {:name "Ivan"} :name "Ivan")]
      #{[1 :name "Ivan"]
        [1 :ref 1]})))

Start by reproducing the test-ref-upsert case with the d/db-with transaction shown in the issue, then trace the nested identity upsert handling. Done means the transaction produces the expected datoms and succeeds regardless of the key order in the transaction map.