Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
When the stream is started initially (see comments in StreamingDatabaseActionContext below). This is needed because we always want to execute the query and buffer the first result page immediately on a subscribe() call even if the Subscriber has not demanded any results. The demand must not be 0 at this point because a transition away from 0 in request() means "restart the continuation".