Tweaks to example httpd configurations.

* Recommend a lifetime of 90 days for static resources, as the
  specification forbids going farther than 1 year.

* Don't recommend setting Cache-Control: public for static resources,
  as this has unforseen consequences wrt caching and cookies on
  Chrome.
Only start session on demand.

This allows some calls, like anonymous API or Atom calls to be cached
more effectively by proxies. Also helps reduce server I/O a little, as
no empty sessions are created for every request made by a cookie-less
user-agent.