Type parameter consistency: I, O, and C

[crankydillo]

Recently, I was trying to get my mind around Retry and ClientConnectionFlow. For the former, the type annotations are good; however, I think they should be changed to match akka-streams conventions (I instead of In and O instead of Out).

I propose C is used for the 'context' type parameter (ClientConnectionFlow[C] instead ClientConnectionFlow[T]). Of course, this is only true if it should have this 'context' meaning. If it really is any type, I would stick with T. However, I wouldn't use T in one place and Context in another.