Fix warnings / deprecations etc. in relation to former Akka 2.5 support

[jrudolph]

Some things could not properly fixed to keep compatibility with Akka 2.5. Since we don't care anymore about 2.5, we can clean things up.

• stream cancellation handling (getting causes in onDownstreamFinish)

[spangaer]

Mhaha, ghosts from the past.

Does sound more like a 1.1 thing though?

[mdedetrich]

Does sound more like a 1.1 thing though?

Yes, these changes should be in 1.1. Stuff like parboiled is an exception because we are not expecting any behaviour change but this would technically be a behaviour change (even if its an improvement).

I will adjust milestone.

[pjfanning]

https://discuss.akka.io/t/handling-upstream-ioexceptions/9285 - stream cancellation handling