Ensure that the detached replicaset objects and their replica objects are GCed after reconfig #570
Description
On reconfig some of the instances or even entire replicasets might be removed. Their replicaset objects and their replicas then won't have any direct references and are supposed to be GCed. But there doesn't seem to be any test covering that.
Lets cover that (with weak refs, see __mode = 'v'). And fix if those objects are leaking.
P.S. Author: @Gerold103. The issue is moved from vshard-ee, so that all of them are in one place, since soon vshard-ee will be archived