Add method for fetching TTL from redis entity

wizard-server/src/main/java/org/cobbzilla/wizard/cache/redis/RedisService.java

@@ -134,6 +134,13 @@ public class RedisService {
         return retry(() -> getRedis().get(prefix(key)), "get");
     }
 
+    /**
+     * @param key
+     * @return TTL in seconds for the given key
+     */
+    public Long get_ttl(@NonNull final String key) {
+        return retry(() -> getRedis().ttl(prefix(key)), "ttl");
+    }
 
     public void set(@NonNull final String key, @NonNull final String value) {
         retry(() -> getRedis().set(prefix(key), encrypt(value)), "set");