mark Datomic calls that make 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))
remove scala.concurrent.blocking markings that are too fine grained
Merge pull request #86 from pellucidanalytics/topic/optimize-blocking

Mark Datomic calls that may result in network access as blocking