Stale ActorRef eviction is broken

ActorRef eviction is designed in the following way:
- keep track of when latest received Pong messages and actor resolutions have occurred
- removing references who haven't been updated beyond a certain threshold from the map
- fire off an attempt to re-resolve these actor refs

The first piece currently isn't functioning as desired. The map is keyed on an Actor's path, and the Pong message is received from the membership actor `/user/sirius/membership`, while the resolution path and the map in the MembershipAgent are keyed on the supervisor's path `/user/sirius`. In short, Pong messages aren't resetting the timeout threshold, and all Refs in the MembershipAgent are recreated (by default) every 40 seconds or so.

This is safe, but it should be fixed in order to work as intended.