message: Move std::function in constructor.
  Recent gcc 12 emits warnings when copying functions (which
  looks like some result if a standard library implementation detail).
  This doesn't happen if we simply std::move the function, so
  let's do that.
diff: @@ -76,7 +76,7 @@ class JsonRpcDispatcher {
  using StatsMap = std::map<std::string, int>;
  // Responses are written using the "out" write function.
  - explicit JsonRpcDispatcher(const WriteFun &out) : write_fun_(out) {}
  + explicit JsonRpcDispatcher(WriteFun out) : write_fun_(std::move(out)) {}
  JsonRpcDispatcher(const JsonRpcDispatcher &) = delete;
  // Add a request handler for RPC calls that receive data and send a response.
  