ike: Use optional jitter to calculate retransmission timeouts

Also adds an optional limit to avoid very high retransmission timeouts
with high numbers of retries.

src/libcharon/sa/task_manager.h

@@ -47,6 +47,11 @@ typedef enum task_queue_t task_queue_t;
  */
 #define RETRANSMIT_TRIES 5
 
+/**
+ * Maximum jitter in percent.
+ */
+#define RETRANSMIT_JITTER_MAX 20
+
 /**
  * Interval for mobike routability checks in ms.
  */