Fix: address corner-case situations in signer set calculations for PoX-5

[aaronb-stacks]

This fixes a handful of corner-case situations for PoX-5 signer set calculations

• Use a Hare quota for distributing "remainder" voting power -- This ensures that if we have many equal weight signers, they don't push the threshold high enough that nobody qualifies.
• Fail fast on empty signing sets -- this should hopefully never occur, but if it does, immediately error so that continued block processing will halt.
• Ensure consistent calculation when PoX-5 activates in a prepare phase -- this makes sure we use the "first pox-5 reward set" logic everywhere.
• Panic if PoX-5 activation is configured for a prepare phase -- this prevents the possibility of the above ever being an issue, and together are a kind of "defense in depth"