=rem #13960 #13989 #13742 #13985  Optimize EndpointWriter

* Replace stash with internal bufferi, j.u.LinkedList
* Replace FSM with become
* Adaptive backoff, important to backoff, but not for too long,
  depends on environment and use case
* Prioritize heartbeat messages from remote watcher and cluster
  failure detector
* Use payload messages as heartbeats for transport failure detector,
  change transport failure detector to be based on absolute timeout,
  see ticket #13989 and #13742
* Log remote disassociate from transport failure detector,
  see ticket #13985
* Add benchmark sample in akka-sample-remote-scala
Merge pull request #2116 from akka/wip-3960-EndpointWriter-stashing-master-patriknw

=rem #13960 #13989 #13742 #13985  Optimize EndpointWriter