Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
TBH I don't think that is an issue at all, assume that the communication between Subscriber and Publisher is done over a network. A number of Request-messages can be in flight while a number of OnNext-messages are in flight in the other direction.

The purpose of this rule is to detect faulty subscribers. And I think it would probably be a good idea to, for each rule in the sec, have a short description of the intent behind the rule.