implement balanced=groupname:lackingcopies

implement balanced=groupname:lackingcopies

Preferred content now supports "balanced=groupname:lackingcopies" to make
files be evenly balanced amoung as many repositories as are needed to
satisfy numcopies.

This implementation could be optimised to only call limitCheckNumCopies
once per file. Currently, it is called in two different places. Or it may
be that it would be better to add a cache to getNumMinCopiesAttr.

It might also be worth implementing :approxlackingcopies, but I'm not sure
if that has a use case. The use case for this seems to be when different
files have different numcopies values.

Sponsored-by: Brock Spratlen