Comment 1 for bug 2069062

As promised by Steve, a branch is available at: https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/miker/lp-2069062-faster-hold-ratio-source

From the commit message:

The Hold Ratio reporting source finds the common ancestor of the hold pickup library and each relevant copy's circ library using two calls to the ancestor.org_unit_ancestor_at_depth() function. This is the most direct way to determine if two org units share a specific common ancestor, but it is also very expensive.

This commit restructures the query by precalculating the set of descendants for each org unit, which allows for a faster test of whether the pickup and circ libs are both in the same descendant list. This is definitionally the same test as having a particular common ancestor, but can be performed MUCH faster.