mod_reqtimeout: follow up to r1621453.

Don't let pipelining checks and keep-alive times interfere
with the timeouts computed for subsequent requests.  PR 56729.


With pipelined requests, the log_transaction hook is called when the request
is destroyed, which may happen after a subsequent request is to be handled on
the same connection.
Move the initialization of the "header" state into pre_read_request() instead,
and get rid of the racy log_trasaction hook.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1689325 13f79535-47bb-0310-9956-ffa450edef68