ab: More accurate stats for time limited (-t) runs.

ab: More accurate stats for time limited (-t) runs.

When ab runs for a limited time, the number of requests configured (or the
default MAX_REQUESTS if not configured) may be lower than than the number
of requests actually/finally achieved, in which case the stats per request
is a window of the actual stats since we can't store all the results.

Rather than taking the last N requests of the run for the stats, do the mean
of all the requests reusing the same window slot, and print that.

If no number of requests is configured for a time limited run, it will stop
only at the end of the time (i.e. MAX_REQUESTS is ignored for the end of
test condition). So MAX_REQUESTS is renamed to TIMED_REQUESTS while at it.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1910517 13f79535-47bb-0310-9956-ffa450edef68