Add batching to Stream TCP

**Issue by [ktoso](https://github.com/ktoso)**
_Monday Nov 30, 2015 at 13:19 GMT_
_Originally opened as https://github.com/akka/akka/issues/19048_

---

The previous stream TCP impl did batching, the current one does not which likely has lessened its performance.

In essence we want to have a generic stage that can collapse multiple incoming ByteStrings into one bigger one. This is likely useful on it's own, but esp. in the Tcp pipeline.

Start by comparing the current Stream TCP implementation with the previous stream TCP implementation, which included batching. Identify how a generic stage could collapse incoming ByteStrings and where it belongs in the TCP pipeline. Done means multiple ByteStrings are batched into a larger one and the TCP stream behavior is covered or validated.

Written by the indexing model from the issue text.