New internal device management

• completely reworked internal device management. Now properly implements the
  listDevices/newDevices/deleteDevices contract recommended by HM's XML-RPC API, and will store local
  device information in a file (default: hm2mqtt.devcache)
• now correctly calls reportValueUsage with count=1 on every channel's datapoint. Note that this may
  result in a large number of pending configurations if devices weren't previously strongly linked to
  the XML-RPC server
• will now attempt to refetch ReGa names every 10 minutes everytime a name fails to resolve.
  When a ReGa fetch worked once (i.e. a ReGa is known to be present), failed names will always be
  published with the retain flag set to false, to avoid the unresolved names ending up in the
  MQTT broker's persistent storage