Update mod_filter to use ap_expr

Advantage: supports more complex expressions while simplifying mod_filter
(c.f. PR 43956 - this means chaining is no longer necessary)
NOTE: this changes FilterProvider syntax, so can't be backported to 2.2.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@642983 13f79535-47bb-0310-9956-ffa450edef68