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
Don't flatten the data for the applications module.
Merge pull request #868 from alphagov/bugfix/applications

Don't flatten the data for the applications module.
Revert "Merge pull request #867 from alphagov/feature/spdy"

This reverts commit 0f433e72ec1c3cf0b6e566228d94b82ee3cded57, reversing
changes made to b9bea851211423656182b6983a876f609d5cbc50.

Seeing if this is causing cheapseats failure in production.
Revert "Revert "Merge pull request #867 from alphagov/feature/spdy""

This reverts commit df5ae65ddaf10d1ff130575159092dcb6388c420.

The build still failed in our CI (but was fine in Travis?). Need to
look for a different culprit.