Support a cluster Observer peer

[tock-ibm]

An observer node is defined in the config, had ID=0, does not take part in consensus. Can only invoke the “Pull Blocks” and “Height” cluster comm services. Can catch up with the cluster and be a hot stand-by Good for bringing a new server to the cluster without reducing fault tolerance or availability.

A subsequent config transaction may change the status of an observer node to a member. The observer node will identify this and promote itself to a member.