=rem #3566 Fix failing RemoteNodeRestartDeathWatchSpec

* Subject actor in freshSystem was not started when the "shutdown"
  message was sent. Solved by retry.
Merge pull request #1704 from akka/wip-3566-RemoteNodeRestartDeathWatchSpec-patriknw

=rem #3566 Fix failing RemoteNodeRestartDeathWatchSpec
=clu #3442 Optimize the vector clock comparison
Merge pull request #1705 from akka/wip-3442-optimize-gossip-comparison-ban

=clu #3442 Optimize the vector clock comparison
!clu #2320 Convert the seen table into something more efficient
Merge pull request #1706 from akka/wip-2320-optimize-the-seen-table-ban

!clu #2320 Convert the seen table into something more efficient