!clu #3683 Change cluster heartbeat to req/rsp protocol

* The previous one-way hearbeat was elegant, but comlicated to
  understand and without giving much extra value compared to this approach.
* The previous one-way heartbeat have some kind of bug when joining
  several (10-20) nodes at approximately the same time (but not exactly
  the same time) with a false failure detection triggered by the extra heartbeat,
  which would not heal.
* This ping-pong approach will increase network traffic slightly, but heartbeat
  messages are small and each node is limited to monitor (default) 5 peers.
=per #3739 Fix broken Javadoc generation

- caused by field named 'native' in LevelDBJournal
Merge pull request #1845 from krasserm/wip-3739-javadoc-gen-broken-krasserm

=per #3739 Fix broken Javadoc generation
Merge pull request #1829 from akka/wip-3683-pingpong-cluster-heartbeat-patriknw

!clu #3683 Change cluster heartbeat to req/rsp protocol
+act #3630 - Only fall back to default ctor if NoSuchMethodException
=act cleanup state parameters in TcpOutgoingConnection

Backport of https://github.com/spray/spray/commit/c480208f5665842103ea6fc8a1b811af38d4a667
Merge pull request #1848 from jrudolph/w/tcp-outgoing-refactoring

=act cleanup state parameters in TcpOutgoingConnection
Merge pull request #1846 from akka/wip-3630-serializer-ctor-fallback-√

+act #3630 - Only fall back to default ctor if NoSuchMethodException
!per #3729 Journaling protocol optimization

- internal batching of individually received Persistent messages
- testing fault tolerance of Processor in presence of random
  * journaling failures
  * processing failures
=act #3733 Re-receive unstashed Terminated messages
Merge pull request #1850 from krasserm/wip-3733-re-receive-unstashed-terminated-krasserm

=act #3733 Re-receive unstashed Terminated messages
Merge pull request #1844 from krasserm/wip-3729-journaling-protocol-optimization-krasserm

!per #3729 Journaling protocol optimization
+per #3746 Remote sharing of LevelDB for testing purposes

Further changes

- remove obsolete identity checks in Eventsourced
- fix wrong serialize-messages config in tests
Merge pull request #1853 from krasserm/wip-3746-shared-leveldb-krasserm

+per #3746 Remote sharing of LevelDB for testing purposes