client: don't hang indefinitive when the connection break

[lynxis]

When the connection closes all client task should terminate. But following example would hang even when the broker closes the connection:

mqcli = MQTTClient(config={'auto_reconnect': False})
await mqcli.connect('mqtt://localhost/')
await mqcli.subscribe([('some/topic', QOS_2),])
message = await mqcli.deliver_message()

The problem lies in the task.set_exception(). It should terminate all pending task. But the problem is Tasks doesn't support set_exception() and this call raise itself an exception.
As the docs state:
"asyncio.Task inherits from Future all of its APIs except Future.set_result() and Future.set_exception()."

However after implementing it. message will return None without any exception.

[lynxis]

This PR only handles the non-auto-reconnecting case.