Preserve closeability of app iterables

PEP 333 (WSGI) says that if your iterable has a close() method, the
framework must call it.

WSGIContext._app_call pulls the first chunk off the returned iterable
to make sure that it gets status and headers, and then it would
itertools.chain() that first chunk back onto the iterable so the whole
body went out. swob.Response.call_application() does it too.

The problem is that an itertools.chain object doesn't have a close()
method, so your iterable's fancy-pants close() method has no chance of
getting called.

This patch adds a slightly smarter CloseableChain that works like
itertools.chain, but has a close() method that calls the underlying
iterables' close() methods, if any.

Change-Id: If975c93f53c27dfa0c2f52f4bbf599af25202f70
Merge "Preserve closeability of app iterables"