Add a timestamp to the Rook request object.

[atheriel]

This PR adds a httpuv.timestamp member containing a timestamp compatible with Sys.time() to the Rook request object. You can use these timestamps it to measure and report on request latency:

httpuv::runServer(
  "127.0.0.1", 8080,
  list(call = function(req) {
    elapsed_ms <- difftime(Sys.time(), req$httpuv.timestamp, units = "secs") * 1000
    list(
      status = 200L,
      headers = list("Content-Type" = "text/plain"),
      body = sprintf("Latency (ms): %.3f\n", elapsed_ms)
    )
  })
)

Importantly, this timestamp is captured when the request is received at the C++ level in the background thread, which has two advantages:

1. We can much more accurately capture the total time the client actually waits for the request to get processed, especially if the R thread is congested or running GC.

2. We can instrument the time it takes before the R-level callback actually runs.

More broadly, the timestamp makes it easier to log measures of request latency in httpuv-based applications, and reduces the overhead of common ways to do so -- e.g. using Plumber hooks.

I'm unsure about whether httpuv.timestamp is a good name, and am happy to take suggestions for improvements.

Also, I have little faith in my C++ (though I've made sure to use C++11-compatible interfaces here).

Finally, if you're interested in R's internal Sys.time() implementation, the source is in time.c:currentTime().

[atheriel]

CI failures seem unrelated.

[wch]

The CI failures are due to RcppCore/Rcpp#1048.

[atheriel]

Tweaked slightly and rebased to fix merge conflicts.

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