allow extraction of PIDs of cluster members
allow extraction of PIDs of cluster members

Subscribing to cluster.MemberStatusEvents was error prone, which
resulted in visibility issues for actors that relied on an up-to-date
topology of the cluster.

By allowing the access of the PIDs of all cluster members of a specific kind,
these actors are able to work properly.

AsynkronIT/protoactor-go,AsynkronIT/protoactor-go,AsynkronIT/gam,AsynkronIT/gam,rogeralsing/gam