async completer / validator?

When using prompt_async, this allows to prompt for user input without blocking, but currently, it doesn't seem to allow using a custom completer/validator. Is that correct?

This would be useful for server-side auto-complete, eg when the object set is big/dynamic.

Eg consider

class AsyncNumberValidator(Validator):

    def __init__(self, session):
        self._session = session
        Validator.__init__(self)

    async def validate(self, document):
        print('AsyncNumberValidator.validate')
        res = await self._session.call(u'com.example.validate_int', document.text)
        print(res)
        return res

used as in

x = await prompt_async('x: ', validator=AsyncNumberValidator(self), patch_stdout=False)

produces:

(cpy360_4) oberstet@office-corei7:~/scm/crossbario/crossbar-examples/prompt$ python client.py 
Connected!
x: 23/home/oberstet/cpy360_4/lib/python3.6/site-packages/prompt_toolkit/buffer.py:1103: RuntimeWarning: coroutine 'AsyncNumberValidator.validate' was never awaited
  self.validator.validate(self.document)
x: 23
RPC succeded: 23 + 1 = 24
(cpy360_4) oberstet@office-corei7:~/scm/crossbario/crossbar-examples/prompt$ python client.py 
Connected!
x: sdfsdf/home/oberstet/cpy360_4/lib/python3.6/site-packages/prompt_toolkit/buffer.py:1103: RuntimeWarning: coroutine 'AsyncNumberValidator.validate' was never awaited
  self.validator.validate(self.document)
x: sdfsdf
2017-02-22T18:02:51 Task exception was never retrieved
future: <Task finished coro=<MyComponent.onJoin() done, defined at client.py:41> exception=ValueError("invalid literal for int() with base 10: 'sdfsdf'",)>

Start by reproducing the prompt_async example with AsyncNumberValidator and inspect how the validator is called in buffer.py, where the RuntimeWarning is reported. Trace prompt_async's handling of custom validators and completers, then verify that asynchronous callbacks are awaited and invalid input is handled without an unawaited-coroutine warning.