Add timeout for callbacks in real-time databroker

[FWuellhorst]

If the execution of callbacks is slower than its creation for a short time-span, e.g. due to network connectivity issues, the callbacks will always keep a certain delay, as the oldest ones are always used first.
An option should exist to automatically exclude callbacks older than some amount of seconds, e.g. older than a minute o.s. If the time exceeds, either a log or an error could be raised.