renderRows() in nodes.js and three places in map.js were using only
n.last_seen to compute active/stale status, ignoring the more recent
n.last_heard from in-memory packets. This caused nodes that were recently
heard but had an old DB last_seen to incorrectly show as stale.
Also adds 29 unit tests for the aging system (getNodeStatus,
getStatusInfo, getStatusTooltip, threshold values).
