Making pool connection cycling async

Nessuno ha ancora preso questa issue.

* **asyncpg version**: 0.14
* **PostgreSQL version**: 9.5/9.6
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: AWS RDS
* **Python version**: 3.6
* **Platform**: linux in docker
* **Do you use pgbouncer?**: Not in this instances
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**:
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: We only use asyncio

We have some application containers that use asyncpg connection pools, every now and again my performance tracing on our application request handlers shows that the `connect` method in the `Pool` takes a significant amount of time (sometimes 150ms) when establishing a connection to our RDS postgres instance (same Region).

The Pool has options to control how often a connection is pro-actively cycled, but my concern is that this results in clients of the pool having to wait for a connection to be established - I would much rather push this into a background async task who's job it is to take connections in/out of service so that inbound http requests to our server never incur the latency of performing the connection themselves.

Is this already doable or would it require some changes to asyncpg?

Inizia leggendo l’entry point Pool.connect e le opzioni esistenti del pool per ruotare proattivamente le connessioni. Traccia quando avviene la connessione per le richieste in ingresso e determina quali modifiche sarebbero necessarie per la rotazione in background; il lavoro è completato quando i client del pool non incorrono più in questa latenza di stabilimento della connessione.

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