Advanced timing metrics

[sagebind]

An initial implementation of request metrics has been delivered for #47, but there are a few other things we could do to enhance the API:

• [ ] Distinguish time taken sending the request and receiving the response. Curl only gives us the "transfer time", which includes both steps. We can calculate these ourselves by recording timestamps in the appropriate callbacks much in the same way that curl does.
• [ ] Use the new curl_off_t variants for getting timing info from curl if possible, as they have higher precision. We will have to consider that old versions of curl do not provide these.