Comment 4 for bug 1234845

I've converted this back to an SQL-language function to reduce overhead and allow the possibility of planning improvements, as well as avoid a little schema churn caused by parameter renaming. It is meant to keep all the improvements from Depesz, and add a little in the way of a ROWS hint for the common case of returning only a few rows instead of the default 1000. See:

http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/collab/miker/lp1234845_ranked_volumes