Clean up performance test

[crawford]

The spirit of the test remains the same - test the throughput of a simple webserver, which has to do some amount of computation, within an enclave - but the implementation has been simplified. The documentation and scripts have also been cleaned up to simplify the setup and execution of the test.

[crawford]

I replaced the encryption with a busy loop to simplify the implementation (and to better control how long each request takes). We aren't benchmarking Golang's encryption implementation, so I wanted to remove that variable from the picture.