Snapshot support (closes #8)

- for LevelDB journal
- for In-memory journal

Further tickets closed:

- Throttled replay support discontinued (closes #87)
- Upper replay limit (closes #89)
- Set processor id before writing messages (closes #90)

Further changes and additions:

- new replay API (ReplayParams)
- Refactor SynchronousWriteReplaySupport
- Refactor AsynchronousWriteReplaySupport
- Rename SnapshottedReplay to IsolatedReplay
- Replay specification
- User guide updated