Move to libevent with libcurl

[jcnelson]

See http://curl.haxx.se/libcurl/c/hiperfifo.html.
See http://curl.haxx.se/libcurl/c/curl_multi_socket_action.html.

We'll need to scale beyond select() one day; might as well use libev or libevent to do so, and update our libcurl multi usage.