fix(transaction): Trigger changes only after COMMIT has been executed

- Fixes #1327
- This caused the dataase query inside QuerySubscription to, on occassion,
  return stale results from the database because the database hadn't
  finished writing. This consequently caused the QuerySubscription to
  build and incorrect result set.

src/flux/stores/database-transaction.coffee

@@ -27,11 +27,11 @@ class DatabaseTransaction
       fn(@)
     .finally =>
       if @_opened
-        @_query("COMMIT")
         @_opened = false
-      global.setImmediate =>
-        for record in @_changeRecords
-          @database.accumulateAndTrigger(record)
+        @_query("COMMIT")
+        .then =>
+          for record in @_changeRecords
+            @database.accumulateAndTrigger(record)
 
   # Mutating the Database