plugin/forward: fix panic when `expire` is configured as 0s (#4115)

Signed-off-by: Ruslan Drozhdzh <rdrozhdzh@infoblox.com>

plugin/forward/persistent.go

@@ -44,7 +44,7 @@ func newTransport(addr string) *Transport {
 
 // connManagers manages the persistent connection cache for UDP and TCP.
 func (t *Transport) connManager() {
-	ticker := time.NewTicker(t.expire)
+	ticker := time.NewTicker(defaultExpire)
 Wait:
 	for {
 		select {