+doc #3656 correcting the TypedActor documentation
=per #3718 Support for native LevelDB
!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.
Merge pull request #1818 from akka/wip-3656-incomplete-TA-docs-√

+doc #3656 correcting the TypedActor documentation
!per #3717 Deletion API for snapshots and persistent messages

- single and bulk deletion of messages
- single and bulk deletion of snapshots
- run journal and snapshot store as system actors
- rename physical parameter in delete methods to permanent
- StashSupport.prepend docs and implementation enhancements
Merge pull request #1839 from krasserm/wip-3718-native-leveldb-krasserm

=per #3718 Support for native LevelDB
Merge pull request #1838 from krasserm/wip-3717-deletion-api-krasserm

!per #3717 Deletion API for snapshots and persistent messages
=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