Optimise validator instance for speed

[renyard]

There are several ways that the validator instance may be able to be optimised, which should be investigated. The following may gain some quick wins:

• Enable SPDY in nginx.
• Enable session cache in nginx.

In future, we may be able to make larger changes to the architecture on both sides, by using WebSockets to keep a connection open and avoid repeated HTTP handshakes & TLS setup.