protected synchronized Set<InternalDistributedMember> getInitialMembers() {
      Set<InternalDistributedMember> initMembers = members;
      members = null;
      return initMembers;
    }
Used to get membership events from our advisor to implement RegionMembershipListener
invocations.

@since GemFire 5.0
geode-core/src/main/java/org/apache/geode/internal/cache/DistributedRegion.java