Measurement for fastestmirror is incorrect when using proxy server

[doug1]

Currently, librepo use libcurl's CURLINFO_CONNECT_TIME to determine the time to connect to remote mirrors. According to https://curl.haxx.se/libcurl/c/curl_easy_getinfo.html this measures "Time from start until remote host or proxy completed". When a proxy server is used, all of my measurements are about 1 millisecond, the time to connect to the proxy server, and not the actual mirror host. Maybe one of the other methods, such as CURLINFO_STARTTRANSFER_TIME would be a better metric.