ap_add_output_filter_handle()

There are many places in the core modules where we do
ap_add_input_filter() or ap_add_output_filter() on a
hardcoded filter name (e.g., "CORE" or "BYTERANGE").
This requires a string-to-filter mapping that wastes
CPU time.  (Even though the string lookup uses a trie
for speed, it still ranks as a big consumer of CPU time
because of the large number of filters added per request.)

The new ap_add_*_filter_handle() functions will allow
us to skip the string-to-filter mapping in cases where
the module adding the filter happens to have the
ap_filter_rec_t* for the registered filter (e.g., because
it's the same module that registered the filter in the
first place).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93039 13f79535-47bb-0310-9956-ffa450edef68