!clu #3683 Change cluster heartbeat to req/rsp protocol

* The previous one-way hearbeat was elegant, but comlicated to
  understand and without giving much extra value compared to this approach.
* The previous one-way heartbeat have some kind of bug when joining
  several (10-20) nodes at approximately the same time (but not exactly
  the same time) with a false failure detection triggered by the extra heartbeat,
  which would not heal.
* This ping-pong approach will increase network traffic slightly, but heartbeat
  messages are small and each node is limited to monitor (default) 5 peers.
Merge pull request #1829 from akka/wip-3683-pingpong-cluster-heartbeat-patriknw

!clu #3683 Change cluster heartbeat to req/rsp protocol