RELAY3: prevent site-down(X) followed by site-up(X)

When a site X has 2 members A and B (and num_site_masters is 1), and A leaves or crashes, then there will be a siteDown(X) notification. As soon as B takes over and becomes site master, there will be a siteUp(X) notification.

Goal: prevent the siteDown/siteUp notifications for the same site, if we know that some other member will take over and become site master, as this might result in unnecessary processing.