Rework voting-only node implementation

Support for voting-only nodes is currently implemented in a plugin and hooks into the cluster coordination subsystem in slightly tricky ways. In particular, voting-only nodes must sometimes win an election so that they can push a fresh cluster state to true master nodes that have stale state. These elections then fail, causing some confusing log messages, and can in theory disrupt elections triggered by true masters too.

We no longer need this feature to be in a plugin, so we can migrate the functionality to the main cluster coordination subsystem, drop the tricky hooks, and prevent these nodes from ever winning elections.