Why is curl_echo() slow sometimes

[jeroen]

The timings of tests of e.g. http2 are 10x slower on Windows and Linux than MacOS: https://cran.r-project.org/web/checks/check_results_httr2.html

I suspect this may be related to: #324: because curl_echo uses the same default port every time, this may cause problems if several packages are checked concurrently (as is the case on cran win/linux) but also maybe a subsequent call will be delayed because the port remains in TIME_WAIT state.

cc @hadley

[jeroen]

I think the delays are caused by httpuv/later, which again may be waiting for the R event loop?

When I first implemented curl_echo, the httpuv::startServer would start a normal server in the foreground. However later versions of httpuv have removed this feature, and now always run a daemonizedServer in the background using later. For me the delays seem to disappear if I downgrade httpuv all the way to 1.3.5, which still had the foreground server.

[jeroen]

Moving to #327