pyon/rpc limits argument size

[jordens]

RPCs with big arguments are broken:

(py35)rj@tito:~/work/nist/artiq/run (master)$ artiq_rpctool localhost 3251 call -t master_schedule foo
"'`seq -s. 20000`'"
Traceback (most recent call last):
  File "/home/rj/src/conda/envs/py35/bin/artiq_rpctool", line 9, in <module>
    load_entry_point('artiq', 'console_scripts', 'artiq_rpctool')()
  File "/home/rj/work/nist/artiq/artiq/frontend/artiq_rpctool.py", line 100, in main
    call_method(remote, args.method, args.args)
  File "/home/rj/work/nist/artiq/artiq/frontend/artiq_rpctool.py", line 79, in call_method
    ret = method(*[eval(arg) for arg in args])
  File "/home/rj/work/nist/artiq/artiq/protocols/pc_rpc.py", line 169, in proxy
    return self.__do_rpc(name, args, kwargs)
  File "/home/rj/work/nist/artiq/artiq/protocols/pc_rpc.py", line 161, in __do_rpc
    return self.__do_action(obj)
  File "/home/rj/work/nist/artiq/artiq/protocols/pc_rpc.py", line 151, in __do_action
    obj = self.__recv()
  File "/home/rj/work/nist/artiq/artiq/protocols/pc_rpc.py", line 146, in __recv
    return pyon.decode(buf)
  File "/home/rj/work/nist/artiq/artiq/protocols/pyon.py", line 190, in decode
    return eval(s, _eval_dict, {})
  File "<string>", line 0

    ^
SyntaxError: unexpected EOF while parsing

On the master:

ERROR:master:asyncio:Task exception was never retrieved
future: <Task finished coro=<Server._handle_connection_cr() done, defined at /home/rj/work/nist/artiq/artiq/protocols/pc_rpc.py:440> exception=ValueError('Line is too long',)>
Traceback (most recent call last):
  File "/home/rj/src/conda/envs/py35/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "/home/rj/work/nist/artiq/artiq/protocols/pc_rpc.py", line 462, in _handle_connection_cr
    line = await reader.readline()
  File "/home/rj/src/conda/envs/py35/lib/python3.5/asyncio/streams.py", line 449, in readline
    raise ValueError('Line is too long')
ValueError: Line is too long

This limits the usability of pdq2 and will break things like other AWGs and things like camera controllers passing images around.