Think about a proper caching strategy

[matze]

Currently, every access to a property will lead to a query of the corresponding hardware property. This increases the perceived latency in some applications, e.g. in the control GUI due to the property tree. Moreover, in 99% of the cases the values will never change except for when the user does so.

We have to think about a general way to allow caching yet always provide the correct value. For example, any temperature property must return the correct value.