Further updates to the script samples

[mstoykov]

#2670 brought up that the script samples aren't working with the latest version.

I did fix some of the easier to fix issues in #2671, but ultimately at least three big issues exist:

1. ./samples/pantheon.js is not working and it likely need to be rewritten from scratch or removed
2. ./samples/websocket.js uses a site that is no longer available - it likely should've always used something hosted by us
3. ./samples/REST_API_testsuite/ was not tested as it's way too big and I do remember it needs work.

Additionally ./samples/http_2.js and ./samples/thresholds_readme_example.js seem to expect some of our test servers to use http 2 ... but they don't :(