1) completely re-implement ab using Perl API (need to add some
   features like arbitrary HTTP request headers)
2) take advantage of HTTP/1.1 keepalives to speed up requests
3) implement with thread support so an execute() can send
   independent sequences of HTTP requests concurrently without
   waiting for all requests to finish in a round
4) ApacheBench.xs badly needs a rewrite
5) need a way to simulate real-world problems: for example
  - slow dialup modem connections
  - user-presses-stop, aka broken socket
6) (re-)sending of individual runs, iterations within a run
