Make Duration::not_zero public.

This is useful for downstream wanting to re-implement `TcpTransport`
This struct is only ever exposed through the `unversionned` module
so its fine to expose this method and remove it in the future

src/timings.rs

@@ -180,7 +180,7 @@ pub struct NextTimeout {
 }
 
 impl NextTimeout {
-    pub(crate) fn not_zero(&self) -> Option<Duration> {
+    pub fn not_zero(&self) -> Option<Duration> {
         if self.after.is_not_happening() {
             None
         } else if self.after.is_zero() {