Since arbiters do not sync data, they should use the commit level when responding to requestVote commands.
This means that they should respond 'nay' to a requestVote when the candidate's optime is behind the commit level. For non-arbiters, the usual logic is to respond 'nay' if the candidate's optime is behind the last op committed on the non-arbiter.
As part of this work, we need to ensure that an arbiter's heartbeats are properly updating its commit level.