nts: rework NTS-KE retry interval

Make the NTS-KE retry interval exponentially increasing, using a factor
provided by the NKE session. Use shorter intervals when the server is
refusing TCP connections or the connection is closed or timing out
before the TLS handshake.

nts_ke_client.c

@@ -393,3 +393,11 @@ NKC_GetNtsData(NKC_Instance inst,
 
   return i;
 }
+
+/* ================================================== */
+
+int
+NKC_GetRetryFactor(NKC_Instance inst)
+{
+  return NKSN_GetRetryFactor(inst->session);
+}