mod_log_config: Backport get_request_end_time().

This makes data consistent if a log format uses
multiple %{...}T and/or %D.
The end time of a request is only taken once and
the same time is used for each log field.

Backport of r979120 (partial) plus r1467765 from trunk
resp. r979120 (partial) plus r1467981 from 2.4.x.

Committed By: rjung
Reviewed By: rjung, trawick, wrowe
Backported By: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1681269 13f79535-47bb-0310-9956-ffa450edef68