core: follow up to r1839997: recycle request filters to a delayed ring first.

We want not only ap_filter_output_pending() to be able to access each pending
filter's *f after the EOR is destroyed, but also each request filter to do
the same until it returns.

So request filters are now always cleaned up into a dead_filters ring which is
merged into spare_filters only when ap_filter_recycle() is called explicitely,
that is in ap_process_request_after_handler() and ap_filter_output_pending().

The former takes care of recycling at the end of the request, with any MPM,
while the latter keeps recycling during MPM event's write completion.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1840002 13f79535-47bb-0310-9956-ffa450edef68