Fetching from pgpool-II hangs indefinitely

まだ誰も着手していません。

* **asyncpg version**: 0.20.1
* **PostgreSQL version**: 12.2
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: No.
* **Python version**: Python 3.7.7 (default, Apr 23 2020, 14:43:26)
* **Platform**: [GCC 8.3.0] on linux
* **Do you use pgbouncer?**: No, I use pgpool-II 4.1.1
* **Did you install asyncpg with pip?**: No, with poetry
* **If you built asyncpg locally, which version of Cython did you use?**: No
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: Yes

When querying the database through pgpool-II it hangs indefinitely, without raising any kind of exception. Connecting to database seems to be working, since for instance invalid password raises an exception:
```
asyncpg.exceptions.InternalServerError: md5 authentication failed
DETAIL: password does not match
```

Steps to reproduce:
```python
import asyncio
import asyncpg

async def run():
conn = await asyncpg.connect(user='user', password='password',
database='database', host='127.0.0.1') # connect to pgpool-II
values = await conn.fetch('''SELECT 1''') # hangs here indefinitely
await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())
```
When connecting to the PostgreSQL directly, the example above works flawlessly.

Log from pgpool-II:
```
May 13 20:56:00 pg01 pgpool[11982]: 2020-05-13 20:56:00: pid 13189: ERROR: unable to read data from frontend
May 13 20:56:00 pg01 pgpool[11982]: 2020-05-13 20:56:00: pid 13189: DETAIL: EOF encountered with frontend
```

提供された Python の再現コードから始め、それを pgpool-II 経由および PostgreSQL に直接対して実行し、動作がどこで異なるかを確認します。pgpool-II のログを根拠として asyncpg connection と conn.fetch path を調査します。SELECT 1 が完了するか、無期限にハングする代わりに有用な exception が発生すれば完了です。

索引モデルが issue の本文から書いたものです。