[feat] store'n forward (ongoing):
when a client disconnect without unsubscribe subscribed topics,
client subscriptions are _transfered_ to offline process
[feat] prevent clients for subscribing multiple times to the same topic.
NOTE: the client is not notified of subscription rejection in agreement
with MQTT 3.1 specifications
[feat] store'n forward (ongoing):
Automatically resubscribe client when connecting if it previously disconnects
without unsubscribing