WIP: add Scala 2.12 and drop 2.10 support

[okapies]

Add Scala 2.12 to crossScalaVersions and remove 2.10 due to dropped 2.10 support in Finagle.

TODO: fix some compile errors.

[mkhq]

@okapies also tried this out. We are moving all our code to 2.12 and this is one of the last blockers. We will spend some time on getting this working in the near future.

[okapies]

I see, thanks!

[mkhq]

Did a small investigation on this. Since the protocol has changed quite some from 0.8.2.1 to 1.0, 2.0 and beyond I would suggest that we start by supporting the message set/record format for 1.0. That would directly make it possible to remove 2.10 and add 2.12.