Attach timestamp to node_events

[kjnilsson]

Currently there is no way for a watcher that receives a node_event to make any judgement on how long ago the state change was observed. Depending on the watcher process implementation it could have spent significant time in the mailbox. If we attach a timestamp to the node_event, receivers of the event can at least evaluate the freshness of the event and possibly choose to ignore it or wait a bit to see if another state change might have been emitted.

A timestamp should be good enough given watchers should only reside on the same erlang node as the aten detector process.

This is a breaking change.