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.
More than one client can't use the same clientID for MQTT connection.
This causes problem in distributed deployments where config is shared
across nodes, as each Minio instance tries to connect to MQTT using the
same clientID.
This commit removes the clientID field in config, and allows
MQTT client to create random clientID for each node.
