allow non-voting p=0 nodes to not require connectivity to other non-voting p=0 nodes

An extension to SERVER-3110, it would be great to support situations where the read-only secondaries only need to be reachable to the primary/voting-secondaries, and not other read-only secondaries that have no chance of becoming master.

At the moment if I try this, the read-only secondaries logs complain about being unable to connect to other read-only secondaries.

An example of such a scenario is where the data is heavily read-only, with 3 instances able to be primary located in a couple DCs. Then dozens more read-only instances scattered across various DCs around the world, providing quick read access to this data. It's impractical to provide the mesh network connectivity currently required between all instances.