mark connection for close after the return from

mark connection for close after the return from
ap_proxy_determine_connection()

before this revision: if there was an existing connection,
ap_proxy_determine_connection() would close it but then clear
the close flag, so it didn't get closed by
ap_proxy_release_connection()

thus, if this child process doesn't use the connection for a
while, the application could be stuck in read() for the same
while

after: ap_proxy_release_connection() will close it after the
request completes

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1032345 13f79535-47bb-0310-9956-ffa450edef68