feat(util): add restore_read function in rpc_message

[Smityz]

Previously, when we use unmarshall like this:

  unmarshall(msg, result);
  unmarshall(msg, result);

The code will core dump

F2020-04-24 17:28:29.746 (1587720509746020513 2ac7) client.THREAD_POOL_DEFAULT0.00002ab200010001: rpc_message.cpp:54:set_read_msg(): assertion expression: r
F2020-04-24 17:28:29.746 (1587720509746053734 2ac7) client.THREAD_POOL_DEFAULT0.00002ab200010001: rpc_message.cpp:54:set_read_msg(): read msg must have one segment of buffer ready
Aborted (core dumped)

and now we can

  unmarshall(msg, result);
  msg->restore_read();
  unmarshall(msg, result);

to avoid this problem.