Make the attribute reading optional while subscribing to an event

In order to boost the starting time of some clients (notably some GUIs), it would be interesting to make the synchronous attribute reading executed while subscribing to an event, optional. In case the target attribute is unreachable (device down, busy or even blocked) or slow to read (slow hardware), the synchronous reading drastically increase the time required to subscribe. Making this reading optionnal would consequently help. In case it is (explicitly) disabled, it will be the developer responsibility to perform the first read - so that the current attribute value is known (and displayed).