Remove work-around for sending to broken connections, see #2909

* Previous work-around was introduced because Netty blocks when sending
to broken connections. This is supposed to be solved by the non-blocking
new remoting.
* Removed HeartbeatSender and CoreSender in cluster
* Added tests to verify that broken connections don't disturb live connection
Merge pull request #1076 from akka/wip-2909-rm-workaround-patriknw

Remove work-around for sending to broken connections, see #2909