Independent delays for reachable and unreachable node notifications

[ralphm]

We recently added a configuration for the delay for reachability notifications. These apply both for nodes becoming unreachable and becoming reachable again. It would good to separate these, so that, for example, one can be notified about nodes becoming reachable again immediately, even though unreachability notifications have a delay of 5 minutes.

Related: #1027, #1039.