fix TODOs
Refactor handling of accepted proposals

Acceptors and Scouts need to keep track of ballot numbers for accepted
proposals, and hang onto only those with the highest ballot number for
each slot.  When the scout has heard from a majority of acceptors,
though, the ballot numbers are no longer required.
don't re-transmit no-ops
replace ldr.scout with just a boolean
remove ldr.commanders
Send CATCHUP messages with a list of slots

This saves a lot of messages, with no other significant change.
chapter work