Added HTTP pipelining support

Netty does not, out of the box support HTTP pipelining.  This adds
support for it, by holding off any writes by a pipelined request until
all requests before it have finished writing their responses.

It is written in Java because it will be contributed to Netty, in the
mean time it can live in Play.
@gissues:{"order":87.5,"status":"inprogress"}