Pool created with min_size loses inactive connections and never re-acquires them

まだ誰も着手していません。

I love `asyncpg`, been using it for years. However recently came across some unexpected behavior with v 0.30.0. It goes a little something like this.

Initialize a pool with a `min_size`:

```python
import asyncio
import asyncpg

pool = await asyncpg.create_pool(min_size=10, max_size=20, **kwargs) # assume kwargs has connection info
print(f'Total: {pool.get_size()}')
# Total: 10
```

All good. But now let's say that nothing happens for over 5 minutes. You know, because `max_inactive_connection_lifetime=300.0` by default. And check again:

```python
await asyncio.sleep(305)
print(f'Total: {pool.get_size()}')
# Total: 0
```

Huh? Why isn't it still 10? No matter what I set for `max_inactive_connection_lifetime` this behavior happens immediately after that time, as long as the time is non-zero. If it's set to `0` then the behavior correctly doesn't happen.

I don't know if this is intentional or not, but as a developer, when I open a database pool with a minimum number of connections I expect that the pool will maintain that minimum, _even if they are automatically closed or timeout_.

asyncpg.create_pool から始め、アイドル接続が閉じられるときに max_inactive_connection_lifetime が min_size とどのように作用するかを調べます。提供された sleep と pool.get_size() のチェックを使って問題を再現します。min_size を設定した pool が、アイドル接続のタイムアウト後もその最小数を維持または再取得できれば完了です。

索引モデルが issue の本文から書いたものです。