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.h

@@ -56,6 +56,10 @@
 #define NKE_MAX_COOKIES                 8
 #define NKE_MAX_KEY_LENGTH SIV_MAX_KEY_LENGTH
 
+#define NKE_RETRY_FACTOR2_CONNECT       4
+#define NKE_RETRY_FACTOR2_TLS           10
+#define NKE_MAX_RETRY_INTERVAL2         19
+
 typedef struct {
   int length;
   unsigned char key[NKE_MAX_KEY_LENGTH];