=rem #3870 Stop re-delivery of system messages when watching non-existing sys

* We don't know the UID so we can't quarantine, but we can stop the endpoint
  writer and drop outstanding system messages
Merge pull request #2007 from akka/wip-3870-stop-redelivery-to-unknown-patriknw

=rem #3870 Stop re-delivery of system messages when watching non-existing sys