Introduced consensus round nonce and reintroduced stage 0. (#160)

* Reintroduced stage 0.
* Introduced round nonce for reduced lcl predictability.

src/crypto.cpp

@@ -131,6 +131,15 @@ namespace crypto
             decoded_pubkey + 1); // +1 to skip prefix byte.
     }
 
+    /**
+     * Generate random bytes of specified length.
+     */
+    void random_bytes(std::string &result, const size_t len)
+    {
+        result.resize(len);
+        randombytes_buf(result.data(), len);
+    }
+
     /**
      * Generate blake3 hash for a given message.
      * @param data String to hash.