pylightning: translate msat input to class Millisatoshi

Rather than using LightningJSONDecoder's implicit "field name and
value ends in msat, try converting to Millisatoshi", we do it to
parameters using type annotations.

If you had a parameter which was an array or dict itself, we don't
delve into that, but that's probably OK.

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

tests/test_plugin.py

@@ -34,7 +34,6 @@ def test_option_passthrough(node_factory):
     n.stop()
 
 
-@pytest.mark.xfail(strict=True)
 def test_millisatoshi_passthrough(node_factory):
     """ Ensure that Millisatoshi arguments and return work.
     """