I learned about AP_MODE_EXHAUSTIVE while reading some of the new filter

I learned about AP_MODE_EXHAUSTIVE while reading some of the new filter
code earlier today.  With this mode, the Perchild MPM can finally be
fixed to work with filters.  I have changed a comment in the core to make
it clear that this mode is required, but I have mentioned how dangerous
this mode is.  Also add a comment to STATUS about my plans.

Hopefully I'll have some time this week to hack through the MPM.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93565 13f79535-47bb-0310-9956-ffa450edef68