Management of default TTL values

As of now, the values of a sensor always use its own TTL. If this value is defined for the sensor, the data will expire according to it, otherwise a default TTL of 0 is used, and the data never expires. The TTL value defined in the Collect Agent configurations is used only for sensors which are not published (no entry in the publishedsensors table).

We should evaluate whether it would be convenient or not to use the Collect Agent's TTL value also when a sensor is published, but has no TTL value associated to it, and not only when the sensor is not published at all.