Making pool connection cycling async

Nadie ha tomado este issue todavía.

* **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?

Empieza leyendo el punto de entrada Pool.connect y las opciones existentes del pool para rotar las conexiones de forma proactiva. Rastrea cuándo se establece la conexión para las solicitudes entrantes y determina qué cambios serían necesarios para la rotación en segundo plano; se considera terminado cuando los clientes del pool ya no incurren en esa latencia de establecimiento de conexión.

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