Allow ClientResponse to define custom flow control

[kxepal]

Also make DataQueue act as like as StreamReader to let FlowControlStreamReader get able to be replaced by FlowControlDataQueue preserving the common behaviour.

[asvetlov]

LGTM

[fafhrd91]

Idea for DataQueue that it can hold any value from parser. and None of empty bytes string are valid values. and with this change i need to treat empty bytes specially as it is not part of what parser emits. even if we decide to accept this change we cannot just return empty bytes, we have define sentinel const and then expose as public api, etc.

example is websockets, servers it receives aiohttp.websockets.Message objects.
https://github.com/KeepSafe/aiohttp/blob/master/examples/wssrv.py#L63

so my opinion is -1

[fafhrd91]

please create PR against master.

just read your comment on 87, we can add special DataQueue that works with bytes objects. and it can inherit from FlowControlDataQueue and it can return b"" instead of EofStream.

[kxepal]

Ah, I didn't mind another cases when DataQueue may be used. Agreed for having special class to handle this case.

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