reassemble twisted websocket messages before passing them to the listener

[bentsku]

Motivation

wsproto emits the data of a message as it arrives: one event per frame of a fragmented message (RFC 6455 section 5.4), and several events for a single frame that is bigger than one socket read. WebSocketChannel queued each of these events as is, so a twisted listener received a partial message from every receive(). For example, a 64 KiB text message sent as one frame arrived as two messages, and a message sent as four 32 KiB frames as six. The ASGI path is not affected, since the ASGI servers reassemble messages before passing them on.

Changes

• WebSocketChannel.dataReceived buffers the data of a text or binary message until wsproto reports message_finished, and queues one event with the whole message. Control frames sent in the middle of a fragmented message (e.g. pings) are still handled as before.

Testing

• New test_receive_large_message (twisted and asgi): the client sends a 1 MiB text and a 1 MiB binary message, each as a single frame. The listener must receive each as one message.
• New test_receive_fragmented_message (twisted and asgi): the client sends a text message as three frames with a ping in between, then a binary message as two frames. The listener must receive two complete messages.
• Both twisted variants fail without the change. The full suite passes (192 tests) and lint is clean.

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