How to remove log listeners for a pool:ed connections.

Nobody has claimed this yet.

According to the [documentation](https://magicstack.github.io/asyncpg/current/api/index.html#connection-pools) the `setup` callback is where you should do stuff like add log listeners to the connection. It works fine but when the connection is release back to the pool I get a warning that it still have a active log listener, which is correct but misleading.

- How/where should I remove the log listener?
- Is there a `release` callback somewhere that I have missed?
- I see that the warning is made using the [`warnings`](https://docs.python.org/3/library/warnings.html) python api. I can just silence it but that might hide actual errors.

**Example code:**
```python
pool = await asyncpg.create_pool(
user='postgres',
setup=lambda conn: conn.add_log_listener(_log_test),
)
```
After I have checked out a connection and released it back this gets printed to stdout/err:
`/opt/python3.7/lib/python3.7/site-packages/asyncpg/connection.py:1295: InterfaceWarning: is being released to the pool but has 1 active log listener`

Start with the connection-pool documentation and the setup callback behavior described in the issue. Inspect asyncpg/connection.py around line 1295 to understand the active-listener warning and trace how connections are released. Done means the listener lifecycle is clear and the misleading warning or missing release mechanism is addressed without hiding unrelated warnings.

Written by the indexing model from the issue text.