Re-use the same temp brigade to read all lines of a request header,

to avoid the overhead of brigade creation and deletion.  (This produced
a 5% reduction in the total CPU usage of a minimalist httpd configuration:
<JHEPKCEMGPKFFDHHDDKDMELFEKAA.bill@wstoddard.com>)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95956 13f79535-47bb-0310-9956-ffa450edef68