Fix reconfig freeze attempting to send to an unbuffered, unread channel (#886)

* Fixes a reocnfig freeze where the reconfig attempts to send to an unbuffered channel with no readers.
Only create stop channel when a DNS goroutine is created, and only send when the channel exists.
Buffer to size 1 so that the stop message can be immediately sent even if the goroutine is busy doing DNS lookups.

lighthouse.go

@@ -262,6 +262,18 @@ func (lh *LightHouse) reload(c *config.C, initial bool) error {
 
 	//NOTE: many things will get much simpler when we combine static_host_map and lighthouse.hosts in config
 	if initial || c.HasChanged("static_host_map") || c.HasChanged("static_map.cadence") || c.HasChanged("static_map.network") || c.HasChanged("static_map.lookup_timeout") {
+		// Clean up. Entries still in the static_host_map will be re-built.
+		// Entries no longer present must have their (possible) background DNS goroutines stopped.
+		if existingStaticList := lh.staticList.Load(); existingStaticList != nil {
+			lh.RLock()
+			for staticVpnIp := range *existingStaticList {
+				if am, ok := lh.addrMap[staticVpnIp]; ok && am != nil {
+					am.hr.Cancel()
+				}
+			}
+			lh.RUnlock()
+		}
+		// Build a new list based on current config.
 		staticList := make(map[iputil.VpnIp]struct{})
 		err := lh.loadStaticMap(c, lh.myVpnNet, staticList)
 		if err != nil {