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

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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_.

Beginnen Sie bei asyncpg.create_pool und untersuchen Sie, wie max_inactive_connection_lifetime mit min_size interagiert, wenn inaktive Verbindungen geschlossen werden. Reproduzieren Sie das Problem mit dem bereitgestellten sleep und den pool.get_size()-Prüfungen. Das Ziel ist erreicht, wenn ein mit min_size konfigurierter Pool dieses Minimum beibehält oder wieder erreicht, nachdem inaktive Verbindungen ihre Zeitüberschreitung erreicht haben.

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