Adding "timeout" on sensor variables or nodes

[danielefrigo]

It would be nice to have a configurable "timeout" on sensors variables (or maybe even at node level) and automatically void variable value after that timeout. The idea is: I know that my Arduino node should send updates every X minutes, if after Y minutes (greater than X) I did not receive a new value, then this means there's some issue on the node or the gw, but anyway that value is not up to date and should be voided.