Add start_time to httpclient and curl_httpclient to better indicate timing reference

I'm using Tornado for some load testing another site (hah) and I found that I have no real way of knowing when a request was executed.  This should get me within a few ms at least.

It'd be cool to have this instead go into time_info as well.  Honestly having the start timestamp and a redundant request_time in time_info for both httpclient and curl_httpclient would be rather handy.