This patch includes the following changes in event store interface
- Removes memory store. We will not persist events in memory anymore, if `queueDir` is not set.
- Orders the events before replaying to the broker.
- The events will be persisted in queueStore if `queueDir` is set.
- Else, if queueDir is not set events persist in memory.
The events are replayed back when the mqtt broker is back online.
