changed difficulty type to string(to avoid uint64 overflow)

src/rpc/core_rpc_server.cpp

@@ -804,7 +804,7 @@ namespace currency
       LOG_ERROR("Failed to create block template");
       return false;
     }
-    res.difficulty = dt.convert_to<uint64_t>();
+    res.difficulty = dt.convert_to<std::string>();
     blobdata block_blob = t_serializable_object_to_blob(b);
 
     res.blocktemplate_blob = string_tools::buff_to_hex_nodelimer(block_blob);

src/rpc/core_rpc_server_commands_defs.h

@@ -791,7 +791,7 @@ namespace currency
 
     struct response
     {
-      uint64_t difficulty;
+      std::string difficulty;
       uint64_t height;
       crypto::hash seed;
       blobdata blocktemplate_blob;