Right now the ReplicaSetMonitor::getHostsOrRefresh only returns the non-preferred option (for PrimaryPreferred and SecondaryPreferred) when the preferred options are completely untargettable.

But, after SERVER-41133, we'll be able to take advantage of the lists of hosts returned in SERVER-41134, and we can weaken our notion of "preferred" from "is targettable" to "has a connection available"

I.e. if matchingHosts =

(where 50% of the time we swap secondary 1 and secondary 2) we have the technology to select a secondary, falling back to the primary, in the order a connection becomes available. In a system with "enough" connections, that should send all traffic to secondaries, and only route to the primary when we have no connections available. I think this weakening is inline with the rest of the work in PM-856