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

Nobody has claimed this yet.

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_.

Start at asyncpg.create_pool and inspect how max_inactive_connection_lifetime interacts with min_size when idle connections are closed. Reproduce the issue with the provided sleep and pool.get_size() checks. Done means a pool configured with min_size retains or re-acquires that minimum after inactive connections time out.

Written by the indexing model from the issue text.