Implementation of shared mode connection pool.

• asyncpg version: 0.21.0
• PostgreSQL version: 12.4
• 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
• Platform: Linux 4.18.0-193.19.1.el8_2.x86_64
• Do you use pgbouncer?: no
• 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?: yes

Currently, asyncpg implements connection pool by blocking the acquired connection in an asyncio.LifoQueue. In aioredis, the connection pool uses so-called shared mode connection which sends the command directly to the connection's StreamWriter and awaits the result from a background asyncio.Future consuming results from StreamReader indefinitely. Is it possible to implement a similar connection pool to optimize the performance of query?

该 issue 将 asyncpg 的连接池和 aioredis 的连接池实现列为起点，但没有提供文件或测试。先阅读这两种池设计，然后确定应如何衡量查询性能和并发命令处理；完成工作需要一个达成共识的共享模式设计、实现范围以及性能证据。