introduce 2 new callback oppurtunities to allow a 3rd party module to do

the actual writing of the log file.
also change buffered writing to make it runtime configurable instead of
compile time configurable.

i'm not 100% on the buffered output, mainly because I'm not sure it worked
in a thread safe way *before* this change


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96538 13f79535-47bb-0310-9956-ffa450edef68