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?

La issue indica il connection pool di asyncpg e l'implementazione del connection pool di aioredis come punti di partenza, ma non indica file o test. Leggi prima entrambi i design dei pool, poi stabilisci come misurare le prestazioni delle query e la gestione dei comandi concorrenti; per completare il lavoro sono necessari un design condiviso concordato per la modalità condivisa, l'ambito dell'implementazione e prove delle prestazioni.