cleanup old-school backward uses of APR_BRIGADE_CONCAT to use

APR_BRIGADE_PREPEND instead, plus optimize away a bunch of repetitive
brigade destructions/creations by hanging onto ctx->saved even after
we empty it out.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94952 13f79535-47bb-0310-9956-ffa450edef68