calculate shared_secret on demand

src/helpers/ContactInfo.h

@@ -9,9 +9,10 @@ struct ContactInfo {
   uint8_t type;   // on of ADV_TYPE_*
   uint8_t flags;
   int8_t out_path_len;
+  mutable bool shared_secret_valid; // flag to indicate if shared_secret has been calculated
   uint8_t out_path[MAX_PATH_SIZE];
   uint32_t last_advert_timestamp;   // by THEIR clock
-  uint8_t shared_secret[PUB_KEY_SIZE];
+  mutable uint8_t shared_secret[PUB_KEY_SIZE];
   uint32_t lastmod;  // by OUR clock
   int32_t gps_lat, gps_lon;    // 6 dec places
   uint32_t sync_since;