immutable peer selection refactoring and enhancements

[kevan]

I've been working on refactoring and improving immutable peer selection. I have several immediate goals for this project.

• Decouple servers of happiness specific peer selection from the more mechanical aspects of peer selection so that it can be easily integrated into SDMF and MDMF mutable files now, and other formats later;
• Address the shortcomings in the servers of happiness file health measure and its current implementation;
• Extend servers of happiness to file check, verify, and repair operations;
• Improve the quality and clarity of the peer selection code, easing future maintenance and experimentation.

These improvements correspond roughly to issues presented in tickets #610, #614, #1115, #1124, #1130, and #1293. Unifying mutable and immutable peer selection is ticket #1057, though I don't expect to address that until after MDMF (#393) is merged and until we're done with this ticket.