Draft: mqtt: Fix that the first notification of some async cmds is sometimes lost

When async cmd is spawned as another process, connection is disconnected, so it can be reestablished again.

If there was a huge delay between process start and first async notification, the existing connection timed out and the notification was not propagated further. Probably this was caused by the fact, that no ping/pongs were sent through the old connection. Sending ping/pings should be restored once the connection is reestablished.

Note that I already tested it on the router and it seemed to be working.

Fixes #267