Prefer a (healthy) hidden secondary to any other node in automatic selection

If a replicaset has a hidden node it was probably intended for backups. At any rate we can be sure it wont have client reads on it, so it's a better choice than other nodes to use as the backup source.

New proposed heuristic:

• Exclude nodes with too much lag (the same way Backup.NodeSuits() already does)
• Hidden nodes should compete to take lock
• If lock not take, other secondary nodes should compete to take lock
• If no winners so far primary should be selected

This proposed improvement overlaps in topic with PBM-488. They should probably be done together.