Stress / long running test of cluster, see #2786

* akka.cluster.StressSpec
* Configurable number of nodes and duration for each step
* Report metrics and phi periodically to see progress
* Configurable payload size
* Test of various join and remove scenarios
* Test of watch
* Exercise supervision
* Report cluster stats
* Test with many actors in tree structure

Apart from the test this commit also solves some issues:

* Avoid adding back members when downed in ClusterHeartbeatSender
* Avoid duplicate close of ClusterReadView
* Add back the publish of AddressTerminated when MemberDowned/Removed
  it was lost in merge of "publish on convergence", see #2779
Fixed startup race in transport adapters #2867
Propagate statusPromise down to the actual change in throttler, see #2877

* added some missing copyright headers
Only client should issue an SSL handshake #2833
Remove LargeClusterSpec, superseded by StressSpec, see #2786
Merge pull request #940 from akka/wip-2786-cluster-stress-patriknw

Stress / long running test of cluster, see #2786
Merge pull request #1004 from drewhk/wip-2833-only-ssl-client-handshakes-drewhk

Only client should issue an SSL handshake #2833
remove extra "the" in docs
Merge pull request #1002 from akka/wip-2877-throttler-ack-patriknw

Propagate statusPromise down to the actual change in throttler, see #2877
Revert "Propagate statusPromise down to the actual change in throttler, see #2877"

This reverts commit 9c1a00d02017a1786820d27a62496afb7e8285ce.
Merge pull request #1005 from akka/wip-2877-revert-patriknw

Revert "Propagate statusPromise down to the actual change in throttler, ...
Remove todo ReentrantReadWriteLock
Merge pull request #1001 from drewhk/wip-2867-throttlertransport-failed-drewhk

Fixed startup race in transport adapters #2867