Enable cache when possible; add new logging level

1) Enable cache for some requests. This reduces number of requests in AT suite with PhantomJS by 40%. 
Time earned is negligible when running on the local machine, but should be more significant when connecting external browsers.

2) Redirect socket.io's "debug" logs into a new level "trace" in attester - socket.io's logging is very extensive and takes over the console when running attester with `--log-level=4`, making it barely usable from our perspective.

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