ab: Add wait time, fix processing time, and output write errors only if

they occured.

§rd patch of P45355.

Proposed by: jailletc36
Reviewed by: jim, rjung

Backport of r1488471 from turnk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1528965 13f79535-47bb-0310-9956-ffa450edef68