mod_reqtimeout: factorize structs and code.

With a bit of macro magic, this is to avoid more code duplication when adding
new stages (next commit will add TLS/handshake timeouts handling in addition to
existing header and body ones).

No functional change here.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1853901 13f79535-47bb-0310-9956-ffa450edef68