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 
A non-member is just a node that's not a part of the group at all; it probably doesn't need to be named here.  Both observers and members receive log broadcasts but only members vote.