Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
As mentioned in the commit message, DistributedBackend/Driver is still experimental (and may even go away instead of being improved) and the current streaming implementation is based on syncrhonous APIs. Since it dispatches to the synchronous APIs of JDBC-based drivers, there is really no correct ExecutionContext better than the global one. To make this production-ready, DistributedDriver would have to be rewritten with streaming at its core.