Snapshotting for HBase journal (closes #98)

- common snapshotting functionality in AsynchronousWriteReplaySupport
- Hadoop FileSystem based snapshot storage
- Fix erroneous non-isolated counter value writes (closes #74)
- Drop useless writer actors (in AWRS)
  - some async database clients will anyway support concurrent writes
  - in all other cases, concurrent writes should be a backend-specific implementation
- extended replay tests