Improve download benchmarking script and add new upload benchmark

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For the reference, here's what the output looks like on my machine:

$ php examples/91-benchmark-download.php http://localhost:8080/10g.bin
Requesting http://localhost:8080/10g.bin…
Headers received 
HTTP/1.0 200 OK
Content-Type: application/octet-data
Content-Length: 10000000000
X-Powered-By: React/alpha
Date: Sun, 16 Feb 2020 20:24:44 GMT

Downloaded 10000000000 bytes in 1.841s => 5430.8 MB/s

$ php examples/92-benchmark-upload.php http://localhost:8080 10000
POSTing 10000 MB to http://localhost:8080
10000000000 bytes in 1.850s => 5405.9 MB/s
The length of the submitted request body is: 10000000000