Memory Leak in SharedMqttClient

[pLeminoq]

I wanted to verify that the previous memory leak (openbase/bco#93) has been fixed and left bco-test --simulate running for about an hour. I found another memory leak, although it is much less grave.

The SharedMqttClient keeps track of how many times a topic has been subscribed to so that it only unsubscribes if none is left. This information is stored in a map but keys are never removed from the map even if the counter goes down to zero. This is a problem since each RPC generates a new topic with a new UID. Thus, this map keeps on growing. After about an hour of runtime it contains almost 160000 elements.