Adds retriers to WTClient and updates code and tests accordingly

Adds a map in the `WTClient` containing the active retriers and their state.
Retriers are active only if they are running or idle.

Adaps the plugin logic to get advantage of this data to better handle retries.
Also adapts tests to check this is consistent.

watchtower-plugin/src/main.rs

@@ -293,36 +293,45 @@ async fn retry_tower(
 ) -> Result<serde_json::Value, Error> {
     let tower_id = TowerId::try_from(v).map_err(|e| anyhow!(e))?;
     let state = plugin.state().lock().unwrap();
-    if let Some(status) = state.get_tower_status(&tower_id) {
-        if status.is_temporary_unreachable() {
-            return Err(anyhow!("{} is already being retried", tower_id));
-        } else if !status.is_retryable() {
+    if let Some(tower_status) = state.get_tower_status(&tower_id) {
+        if let Some(retrier_status) = state.retriers.get(&tower_id) {
+            if retrier_status.is_idle() {
+                // We don't send any associated data in this case given the idle retrier already has it all.
+                state
+                    .unreachable_towers
+                    .send((tower_id, RevocationData::None))
+                    .map_err(|e| anyhow!(e))?;
+            } else {
+                // Status can only be running or idle for data in the retriers map.
+                return Err(anyhow!("{} is already being retried", tower_id));
+            }
+        } else if tower_status.is_retryable() {
+            // We do send associated data here given there is no retrier associated to this tower.
+            state
+                .unreachable_towers
+                .send((
+                    tower_id,
+                    RevocationData::Stale(
+                        state
+                            .towers
+                            .get(&tower_id)
+                            .unwrap()
+                            .pending_appointments
+                            .iter()
+                            .cloned()
+                            .collect(),
+                    ),
+                ))
+                .map_err(|e| anyhow!(e))?;
+        } else {
             return Err(anyhow!(
                 "Tower status must be unreachable or have a subscription issue to manually retry",
             ));
         }
-
-        state
-            .unreachable_towers
-            .send((
-                tower_id,
-                RevocationData::Stale(
-                    state
-                        .towers
-                        .get(&tower_id)
-                        .unwrap()
-                        .pending_appointments
-                        .iter()
-                        .cloned()
-                        .collect(),
-                ),
-            ))
-            .map_err(|e| anyhow!(e))?;
-
-        Ok(json!(format!("Retrying {}", tower_id)))
     } else {
-        Err(anyhow!("Unknown tower {}", tower_id))
+        return Err(anyhow!("Unknown tower {}", tower_id));
     }
+    Ok(json!(format!("Retrying {}", tower_id)))
 }
 
 /// Forgets about a tower wiping out all local data associated to it.