Support "singleton" option

[KenBirman]

We currently have a seemingly unnecessary restriction: Any Derecho top-level group must have >= 2 members. Yet for our protocols, there really is no reason that a top level group of 1 couldn't be supported. I am proposing that we remove this unnecessary limitation, but then we need to retest carefully lest something break. Edward gave some examples of things that could go wrong during subgroup and SST construction, and that need to be checked.

Rationale: Newcomers learning Derecho and Cascade might not initially have full access to a suitable cluster or cloud, or might not know how to set up firewalls to allow TCP and RDMA connections between distinct nodes. Rather than viewing these as required prior knowledge, we lower the barrier to first use of the technology by allowing such students or developers to run a single-instance system on their own laptop or desktop, enabling application development and debugging (even with 1 node, functionality and APIs would be unaffected). In singleton mode (1) A single top-level leader would suffice; (2) All subgroups would be initialized with this one node as their sole member; (3) localhost would be used for external client connectivity. Later, with a working application, the developer could tackle multi-node setup, firewalls and other such issues.