util_filter: split pending filters ring in two: input and output ones.

Pending input and output are now maintained separately in respectively
c->pending_input_filters and c->pending_output_filters, which improves
both performances and debug-ability.

Also, struct ap_filter_ring is made opaque, it's only used by util_filter
and this will allow us to later change it e.g. to a dual ring+apr_hash to
avoid quadratic search in ap_filter_prepare_brigade().

MMN major bumped due to the change in conn_rec (this is trunk only code
anyway for now).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1839933 13f79535-47bb-0310-9956-ffa450edef68