Remove PROPOSAL and VOTE generics from CommunicationChannel; Add COMMITTABLE generic to VoteType

[elliedavidson]

This PR:

• Removes PROPOSAL and VOTE generics from the CommunicationChannel trait
• Adds COMMITTABLE generic to VoteType, since each vote needs to be over a specific committable type
• Adds placeholder Accumulator2 code for the new accumulator that will be certificate-based instead of exchange based.

NOTE: There are still TODO ED notes in the code. We can merge this in now to get the above changes in, or we can wait until I finish the accumulator logic. I don't mind either way.