* Now supports HTTP/1.1 persistent connections with pipelining.
  This speeds up chain enumeration on large files by almost a
   factor of 2 over http, and by roughly a factor of 4 over https.
  The difference between http and https is now much smaller.
* Add timeouts to all the socket I/O.
* Estimate the number of available bytes to read and use it when
   making connection re-use decisions.
* Add support for https with proxies using HTTP/1.1 CONNECT
   tunnels.
* Fix TLS session re-use (it requires clean shutdown).
* Various other code re-organization and minor improvements.