Pool connect ignores DNS ttl

还没有人认领这个 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?**: Using 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

After performing a failover/reboot of an AWS RDS instance asyncpg does not appear to reconnect gracefully - it looks like the code in `Pool` caches the IP of the PG instance after dns resolution and does not respect whatever TTL we might have set on the cname that points at our master: https://github.com/MagicStack/asyncpg/blob/master/asyncpg/pool.py#L133

从 asyncpg/pool.py 第 133 行附近开始，跟踪 Pool 如何解析和复用 PostgreSQL 主机。检查报告中的 AWS RDS 故障转移场景，以及在 asyncio 下如何处理 DNS 变更。完成标准是：故障转移后，pool 会重新连接到更新后的地址，而不是保留旧的已解析 IP，并为该行为提供回归覆盖。

由索引模型根据 Issue 内容生成。