Snapshot requests
Drop useless writer actors

- some async database clients will anyway support concurrent writes
 - in all other cases, concurrent writes should be a backend-specific implementation
Fix erroneous non-isolated counter value writes

- closes #74
Snapshotting for HBase journal

- snapshot support in AsynchronousWriteReplaySupport
- in-memory snapshot storage at the moment
- replay tests with upper limits are still failing
Snapshotting for HBase journal

- snapshot support in AsynchronousWriteReplaySupport (extended)
- replay tests with upper limits now working
- replay tests with snapshots now working
- in-memory snapshot storage at the moment
Extend replay tests
Snapshotting for HBase journal

- write snapshots to Hadoop filesystem
- closes #98