Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
Future.apply works for materialized results (and if you go back in the history of this PR you'll see that the original version used it) but it doesn't work for the continuation handling required for streaming. An EC was my first idea here but some drivers need to use `blocking` (DistributedBackend) while it would be unnecessary overhead (JdbcBackend) or even the wrong thing to do (HeapBackend) for others.