stops working(?) if server is not reachable for longer(?) time

[st0ff3r]

has to check when it happens

[st0ff3r]

can be provoked by DROP'ing packets with iptables

[st0ff3r]

posible fix in b832239

tuanpmt/esp_mqtt@0f980bf

"If the current connection is terminated unexpectedly (e.g. because the
WiFi connection is dropped) the disconnect callback is not called. This
means that reestablishing the existing connection is not possible any
more.

Avoid this issue by forcefully removing the existing connection before
initiating the reconnection. Calling MQTT_Disconnect and waiting for
the disconnection callback should still be performed for intentional
disconnections."