Faust breaks if input messages have extra fields that we don't specify in our model about faust

[danielko-robinhood]

Stack trace for schema not exactly match. This caused the faust app crash and restart:

Traceback (most recent call last):
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/serializers/registry.py", line 128, in loads_value
    return typ(**v)
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/models/record.py", line 155, in __init__
    self._init_fields(args, kwargs, strict=__strict__)
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/models/record.py", line 181, in _init_fields
    ', '.join(sorted(extraneous))))
TypeError: MainStreetOrder got unexpected argument: response_category

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/agents/agent.py", line 303, in _execute_task
    await coro
  File "/home/robinhood/env/lib/python3.6/site-packages/requiem/agents.py", line 22, in filter_order_updates
    async for order in orders:
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/streams/stream.py", line 632, in __anext__
    value = await self._on_message()
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/streams/stream.py", line 552, in on_message
    value: Any = await get_next_value()
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/channels.py", line 394, in __anext__
    return await self.queue.get()
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/utils/futures.py", line 220, in get
    return await super().get()
  File "/home/robinhood/python-3.6.3/lib/python3.6/asyncio/queues.py", line 175, in get
    return self.get_nowait()
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/utils/futures.py", line 231, in get_nowait
    raise self._errors.popleft()
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/topics.py", line 125, in decode
    value_type, message.value, serializer=value_serializer)
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/serializers/registry.py", line 134, in loads_value
    str(exc)).with_traceback(sys.exc_info()[2]) from exc
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/serializers/registry.py", line 128, in loads_value
    return typ(**v)
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/models/record.py", line 155, in __init__
    self._init_fields(args, kwargs, strict=__strict__)
  File "/home/robinhood/env/lib/python3.6/site-packages/faust/models/record.py", line 181, in _init_fields
    ', '.join(sorted(extraneous))))
faust.exceptions.ValueDecodeError: MainStreetOrder got unexpected argument: response_category
[2018-01-23 05:15:37,049: INFO]: [^---Agent*: requiem.a[.]filter_order_updates]: Starting...
[2018-01-23 05:15:42,050: INFO]: [^---OneForOneSupervisor]: Restarting dead <Agent*: requiem.a[.]filter_order_updates>! Last crash reason: ValueDecodeError('MainStreetOrder got unexpected argument: response_category',)
[2018-01-23 05:15:42,051: INFO]: [^--Stream: <Topic: mainst_order_updates>]: Starting...```

[danielko-robinhood]

To fix this, I'll have to add the corresponding fields whenever the input Kafka message adds a new field:

https://github.com/robinhoodmarkets/requiem/commit/f915cf31dbfb37ac0227850ab70e0ce5c5c710a8#diff-0f63698bab3e064e4f23917b871a446cR46

[vineetgoel]

@ask looks like we should allow when incoming field has extra fields. Our model deserialization should not break on backwards compatible changes.