This PR adds ahttpuv.timestamp member containing a timestamp compatible withSys.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")*1000list(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 inhttpuv-based applications, and reduces the overhead of common ways to do so -- e.g. using Plumber hooks.

I'm unsure about whetherhttpuv.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 internalSys.time() implementation, the source is intime.c:currentTime().