Sessions on arbiters accumulate and cannot be cleared out

We allow the creation of logical sessions on arbiters. These sessions are distinct from sessions on the rest of the replica set or sharded cluster; they are strictly local.

However, the logical session reaper does not work on arbiters. Since SERVER-40441, it is disabled explicitly. Before that, it would fail because config.system.sessions does not exist on arbiters (and even if you created it, it wouldn't be sharded on a sharded cluster). This results in eventually running out of space in the session cache, if any are being created.