Consider making the following changes to the way we record depths for memberships after investigating how depths are used in the API and UI.

1. Currently depths start at 0. Does it make more sense to start them at 1? That way all immediate memberships will have a depth of 1.
2. Composite memberships always have a depth of 0. Does it make sense for composite memberships to have the same depth as the depth to the factor group? Note that this might mean that we have to record parent memberships for composite memberships. Also, for intersect composites, there will be 2 membership entries for each member. And for union composites, there may be one or two membership entries for each member.