Add support for Finagle Framework

[JonasKunz]

Finagle is a reactive scala framework for RPC. It is internally based on netty. Finagle uses future composition for building processing logic and therefore doesn't work well with our standard thread-based context propagation.

In order to provide a useful instrumentation, we need to cover at least the following features:

• [ ] Implement context propagation across finagle futures (most likely via contexts )
• [ ] Add instrumentation for the finagle HTTP server
• [x] Add instrumentation for the finagle HTTP client