Memory leak with Pool usage

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* **asyncpg version**: 0.27
* **PostgreSQL version**: 14
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: no
* **Python version**: 3.8, 3.10 and 3.11
* **Platform**: docker images python:3.x-slim-buster
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: only asyncio

When we use a Pool, the memory usage of our process keeps increasing slowly with the number of queries. After 1000 queries, it is ~10MB and this increases with the number of queries executed on connections acquired from the pool. This remains after the max_inactive_connection_lifetime has passed with no activity, with explicit garbage collection and even after we close the pool. We can reproduce this with a psutil measurement and observe it in production. When we replace the usage of Pool with our own creation/cleanup of asynpg Connections, the issue dissapears.

Pool created with the following arguments:

```
asyncpg.create_pool(
max_inactive_connection_lifetime=300,
command_timeout=30,
min_size=0,
max_size=5
)
```

Inizia dal punto di ingresso asyncpg.create_pool con gli argomenti del pool indicati e riproduci l’aumento graduale della memoria utilizzando la misurazione psutil descritta. Confronta le connessioni del pool con connessioni asyncpg dirette, anche dopo max_inactive_connection_lifetime, la garbage collection esplicita e la chiusura del pool. Il lavoro è completo quando la riproduzione non mostra più una crescita correlata al numero di query.

Scritto dal modello di indicizzazione a partire dal testo della issue.