Implement StreamReader.readuntil() method

[asvetlov]

Should be mimic to asyncio.StreamReader counterpart: https://docs.python.org/3/library/asyncio-stream.html#asyncio.StreamReader.readuntil

[asvetlov]

Looks like nobody need it.
Feel free to reopen the issue with the use case

[yapith-suplari]

@asvetlov I described my issue/use-case in #3881, but I'll summarize it here:

I'm pulling data from web services that don't provide response content-length and doesn't not have explicit EOF character. Under high concurrent network load, I'm seeing await read() returns partial data sometimes. I suspect that it catches its internal buffer empty and terminates prematurely. (Is this even possible? Let me know if I'm making bad assumptions here)

readuntil allows me to seek an explicit end-of-stream indicator if known and make aiohttp more reliable