Detect failure when no heartbeats sent, see #2907

* Subscribe to InstantMemberEvent and start heartbeating when
  InstantMemberUp. Same for metrics.
* HeartbeatNodeRing data structure for bidirectional mapping of
  heartbeat sender and receiver. Not using ConsistentHash anymore.
  Node addresses are hashed to ensure that neighbors are spread out.
* HeartbeatRequest when receiver detects that it has not received
  expected heartbeats.
* New test InitialHeartbeatSpec that simulates the problem
* Add/remove some related conf properties
* Add some more logging to be able to diagnose eventual problems
* Explicit config of nr-of-end-heartbeats
Merge pull request #1029 from akka/wip-2907-heartbeats-patriknw

Detect failure when no heartbeats sent, see #2907