perf: try to reduce ping overhead

[SukkaW]

The PR changes two things:

• /ping now also supports HEAD request, and it will respond with HTTP 204 with an empty response body
• The front-end now uses HEAD requests. And instead of using json() (which has JSON parse overhead) new implementation uses arrayBuffer()
  • Technically, we don't have to consume the body, but on some browsers (namely Safari), this would cause memory leaks (just like on Node.js). But since we return an empty response from the server, this should not cause any performance overhead anyway.
  • Also, with the PR, it now calculates the RTT before consuming the body.