Support multiple-packet messages

* Put the length of the JSON file in front, and read byte-by-byte until
  we see the open curly brace. Then read length-1 bytes.
* No other changes to the interface were made.
* Try this: in one console, run ./server.py; in another console, open
  the Python interpreter, and do

  >>> from t import *
  >>> t()
  >>> tt()