Use SPDY to connect to backing services

Spotlight currently makes a lot of backing service requests over SSL. We
should see an improvement in our response times by using SPDY to connect
to each domain/port combination over SPDY, and only pay the overhead of
setting up a single TCP connection to have a conversation over TLS.

Currently, a request to render HTML might make 10 requests. Default node
settings will allow up to 5 connections to be made to a host / port
combination, so we will have the overhead of setting up 5 TLS
connections, then hopefully see connection keep alive giving a slight
benefit.

This change needs benchmarking and profiling to ensure that we’re
using SPDY correctly.

Can be traced in wireshark / tcpdump with ip.addr == 37.26.89.53 on
preview.
Merge pull request #867 from alphagov/feature/spdy

Use SPDY to connect to backing services