Making pool connection cycling async

まだ誰も着手していません。

* **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?

まず、Pool.connect のエントリポイントと、接続を事前にローテーションするための既存のプールオプションを読みます。受信リクエストに対する接続確立がいつ発生するかを追跡し、バックグラウンドでローテーションするにはどのような変更が必要かを判断します。完了の条件は、プールクライアントが接続確立のレイテンシを負担しなくなることです。

索引モデルが issue の本文から書いたものです。