Add support for non-voting members

[btmorr]

Add a state (or other mechanism) by which a node may be a non-voting member of a cluster. These nodes accept log appends and commits from leaders, but cannot become leaders and do not participate in elections (thus do not count in the tally of how many nodes are required for a quorum).

This is used in a few circumstances (list not exhaustive):

• Additional read members that do not increase guaranteed fault tolerance but can still share the load of servicing read requests
• Nodes that are going to become voting members of the cluster, but need to catch up with log history without becoming an election bottleneck
• A node that was a member of a cluster, and stopped being a member through dynamic configuration change, but happened to be the leader through the transition (e.g.: the cluster might have a non-voting leader for a short time after configuration change)

Note:

• Related to Add ability to dynamically change cluster membership #81