fix and update code to run on current library versions

[mdecker]

Hi, thanks for your code. I needed to make some changes to have it run on my system. I assume it might be because of changes to the mqtt library or server over the past 3+ years. In any case: This code works for me. So feel free to merge it.

Key changes I made:

• move subscribe to on_connect
• only subscribe to topics actually required
• fix number of expected topics
• move QoS setting and number of expected topics to top-level scope
• only print performance data when all required data is available (avoids KeyError)

tested with: mosquitto 2.0.11, paho-mqtt 1.4.0, Debian 10