Bulb-channels

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* Wraps session save calls in ``database_sync_to_async()``, for compatibility
with Django 3.0's ``async_unsafe()`` checks.

* Drops compatibility with all Django versions lower than 2.2.

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* Adds compatibility with Python 3.8.

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* Adjusted ``AsgiHandler`` HTTP body handling to use a spooled temporary file,
rather than reading the whole request body into memory.

As a result, ``AsgiRequest.__init__()`` is adjusted to expect a file-like
``stream``, rather than the whole ``body`` as bytes. Test cases instantiating
requests directly will likely need to be updated to wrap the provided body
in, e.g., `io.BytesIO`.

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* Updated requirements for ASGI v3 and Daphne 2.3.

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* HTTP request body size limit is now enforced

* database_sync_to_async now closes old connections before it runs code

* Auth middleware closes old connections before it runs

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* HttpCommunicator now extracts query strings correctly

* AsyncHttpConsumer provides channel layer attributes

* Prevent late-Daphne import errors