Simplify stream classes

[asvetlov]

• drop ChunksQueue -- not used by aiohttp code
• drop FlowControlChunksQueue -- not used by aiohttp code
• Merge FlowControlStreamReader into StreamReader, drop FlowControlStreamReader. The reader without flow control is not used by aiohttp, FlowControlStreamReader is too long name. Getting rid of await super() calls may boost performance a little.
• Add hysteresis to StreamReader to prevent flipping between paused and resumed states too often.

[kxepal]

FlowControlChuckQueue is useful when server emits item-per-chunk - that's quite handy to process. CouchDB does that for instance. Let me check if this approach is still actual.

[asvetlov]

If CouchDB needs FlowControlChuckQueue -- please copy the implementation into the project.
Supporting not used classes is a burden, you know.

[kxepal]

Not a problem, I know (: Probably, that even wouldn't be need after switching to ijson.

[kxepal]

Just wanted to describe the case when that flow was useful. Some else users eventually may use it as well.

[asvetlov]

Fixed by @2256

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new