Fixed problems handling passive connections in Remoting #2878

- Fixed potential race in EndpointWrite when TakeOff is processed
- Reenabled accidentally disabled gremlin due to changed defaults
- Fixed EndpointRegistry incorrectly handling passive connections
Update akka-docs/rst/general/actor-systems.rst
Shut down both client and server channels. See #2862
Clean up network connections from the test coordinator and player. See #2862
Merge pull request #1037 from akka/wip-2862-leaking-netty-threads-ban

Leaking Netty threads causes file handle exhaustion
Merge pull request #1031 from drewhk/wip-2780-removal-of-old-remoting-drewhk

Removed remaining parts of the old remoting #2780
Preserving Envelopes in UnstartedActorCell as requested by Mr Pink
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
Unique barriers for each round in StressSpec.exerciseSupervision, see #2905

* Looks like the issue was caused by a mixup of barriers and/or name
  of the result aggregator actor
Merge pull request #1032 from akka/wip-2905-StressSpec-exercise-supervision-patriknw

Unique barriers for each round in StressSpec.exerciseSupervision, see #2905
Merge pull request #1034 from akka/wip-retain-envelope-√

Preserving Envelopes in UnstartedActorCell as requested by Mr Pink
Style change of def starting with if

* When a def starts with if and is not a oneliner the if
  should be on a new line.
* The reason is that it might be easy to miss the if when
  reading the code.
Merge pull request #1033 from akka/wip-def-if-patriknw

Style change of def starting with if
Merge pull request #1036 from derekmahar/patch-4

Update akka-docs/rst/general/actor-systems.rst
Making test more DRY. See #2862
Terminate zeromq context after actors have terminated. See #2838
Merge pull request #1039 from akka/wip-2838-zeromq-test-shutdown-race-ban

Terminate zeromq context after actors have terminated