Group notified on member unmanaged

- and member notified on group unmanaged.
- done automatically, without need for registering listeners.
- will be generalised to “relationships” at some point.
Deprecate DynamicGroup.stop
Deserialize: discard dangling ref in ImmutableSet

- If didn’t then it would fail to deserialise, which would cause 
  entire file to fail to deserialise.
  The dangling ref will have been logged plus handled by the 
  RebindExceptionHandler, so mostly harmless ignoring it here.
Fix logging on Effectors.invoke
Tidy QuarantineGroup.expungeMembers
This closes #24.