bpo-30064: Fix slow asyncio sock test (GH-20868)

Using a log2n way to fill a much smaller buffer, and receiving in a cleaner way with EOF.

The failing test was reproducible using the following command thanks to @aeros :

```bash
./python -m test test_asyncio.test_sock_lowlevel --match test_sock_client_racing -j100 -F -v
```

According to test results, we may still need to bump the timeout:

https://github.com/python/cpython/blob/5aad027db9618f22f6fa2274e05dd50f928d2ed7/Lib/test/test_asyncio/test_sock_lowlevel.py#L256-L257