Implement graceful removal of data store actors

[Neverlord]

Masters and clones no longer use direct actor-to-actor communication after redesigning store communication in the ALM branch. Instead, the new channel abstraction enables them to communicate over the pub/sub layer. However, there is currently no graceful way to sever a channel "connection" gracefully. Channels only remove participants after timeouts.

Blocks #126, because we can't re-attach a clone if we can't remove it from the master in the first place.