lightning.py: parse multiple JSON RPC commands accurately.

We need to keep the remaining buffer, and we need to try to parse it
before we read the next.  I first tried keeping it in the object, but
its lifetime is that of the *socket*, which we actually reopen for
every command.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -611,8 +611,9 @@ def test_multirpc(node_factory):
 
     sock.sendall(b'\n'.join(commands))
 
+    buff = b''
     for i in commands:
-        l1.rpc._readobj(sock)
+        _, buff = l1.rpc._readobj(sock, buff)
     sock.close()