* Add acking mechamism for shared dispatch
For QoS0 messages, no acking
For QoS1/2 messages, 'ACK' at any of events below:
- ACK when QoS is downgraded to 0
- Message is sent to connection process
'NACK' at any of events below:
- Message queue is full and the receiving session starts to drop old messages
- The receiving session crash
Upon 'NACK', messages are dispatched to the 'next' subscriber in the group,
depending on the shared subscription dispatch strategy.
Before this change, eqmx_mock_client uses a shared ets table
to store last received message, this causes troulbe when we
want to start / stop two or more clients in one test case
the ets table gets owned by the first spanwed client and
gets closed when the owner client dies.
Now it keeps the last received message in process state
and a gen_server call is added to retrieve it for verification
Along with this change in emqx_mock_client, it made possible
to write test case to verify the actual subscriber pid
used in shared subscription strategy, so test cases were
added (and modified) to verify different strategies
turtled