=per #3718 Support for native LevelDB
!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 remote-tracking branch: upstream/master