Remove joint consensus (ProposedMembers) support.

This was present in the original raft paper but was removed from the final
thesis because simpler protections suffice as long as only one node is added
or removed at a time.

Use the term "observer" consistently for non-voting members.

@spencerkimball 
Can't we enforce that only one node may be a non-voting member at a time? Also, nodes could keep a flag indicating that a change membership op is currently pending, and then return an error if new calls to changeGroupMembership are made while the flag is true.