Mark Datomic calls that may result in network access as blocking

Make use of [scala.concurrent.blocking](http://www.scala-lang.org/api/current/scala/concurrent/package.html#blocking) (see [scala-user discussion](https://groups.google.com/d/topic/scala-user/XIhVwRZiucE/discussion)).

(Query and entity access seem to be the crucial cases to cover. Not sure quite how to handle raw index access.)