The previous staticweb had the silly problem of global state for
concurrent requests. This fixes that.
Also, the WSGI spec indicates start_response might not be called
right away and is only guaranteed to be called just before the first
"chunk" of the output iterator returns. This fixes StaticWeb's
previously incorrect assumption.
A thorough review of this code would be much appreciated; I've
messed it up enough times to not be completely trusted with it.
Change-Id: Ie751c24e21db7a884a83a731fbf2f4309437302c