Use ActorSelections for remote nodes in Membership

Sirius currently attempts to resolve ActorRefs for each node in the MembershipActor, and store those refs for use by any actor within the system for communicating with remote nodes. These refs aren't necessarily long-lived, and maintaining them is a pain.

Instead of resolving ActorRefs and maintaining them in the MembershipMap, we should instead store ActorSelections. When an actor needs to initiate a session of communication with a remote node, it can resolve a ref from the selection -- or, more likely, simply send a message via the selection and implicitly get the ref for future use when the remote node responds.