core: integrate data_in_{in,out}put_filter to ap_filter_{in,out}put_pending().

Straightforward for ap_filter_input_pending() since c->data_in_input_filter is
always checked wherever ap_run_input_pending(c) is.

For ap_filter_input_pending(), it allows to set c->data_in_output_filter in
ap_process_request_after_handler() to avoid an useless flush from mpm_event.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836239 13f79535-47bb-0310-9956-ffa450edef68