Http pipelining support

Replaces #898. Uses a new library at the typesafe hub. This library [1] provides pipelining support to Netty in the form of a handler.

The changes here constitute the changes required to Play in support of the new library. Some re-factoring of Play's upstream handler has been done in the process.

Given that the new library exists as a snapshot (at least for now), I've also enhanced the build to consider snapshots during the building of Play in snapshot form. Snapshots will not be resolved though when distributing Play - this is what we want of course.

[1] https://github.com/typesafehub/netty-http-pipelining
@gissues:{"order":50,"status":"done"}