Issue 1461 - rough idea how to select mirror vdev member by estimated queue service time

Warning upfront: I doubt the pull request will compile, apart from 'Hello World' i never coded C - so please **digest this as pseudocode** and indulge me for serving this **untested**

Idea is to track average request duration (256 recent requests) for each vdev and select children for ready by queue lenght * average request duration.