Sensors unable to provide their state when instantiated

[tobie]

Some HW sensors on some platforms are unable to provide their state upon instantiation and sometimes take a very long time to provide any reading whatsoever.

This is related to an old issue (see #87), was a concern in previous APIs and this behavior is currently in violation of the spec.

Perhaps the spec should special case such sensors and force them to advertise their inability to provide the current state upfront (e.g. via a dedicated event?)

[alexshalamov]

@pozdnyakov Moving this to Level2. I'm not sure what sensors are unable to provide their state in a reasonable time, needs research and probably would be good fit for 'discovery' related APIs.