[SoC2020] PostgreSQL: add io_uring support

# 项目名称/Project Title
PostgreSQL: 增加 io_uring 支持
PostgreSQL: add io_uring support

# 项目描述/Project Description
PostgreSQL 是一款强大的开源对象-关系数据库服务器（ORDBMS），特点是可靠，特性健壮以及高性能，且支持跨平台。PostgreSQL 的 Slogan 是“世界上最先进的开源关系型数据库”，其官网链接：https://www.postgresql.org/

本项目计划在 PostgreSQL 中增加对 io_uring 的支持，以优化异步 io 模型的性能，目前社区也有在做这方面的尝试。参考资料：https://anarazel.de/talks/2020-01-31-fosdem-aio/aio.pdf

PostgreSQL is a powerful, open source object-relational database system with over 30 years of active development that has earned it a strong reputation for reliability, feature robustness, and performance. It supports running corss-platform. The slogan of PostgreSQL is "The World's Most Advanced Open Source Relational Database", as seen on the offical website is https://www.postgresql.org/.

This project wants to add io_uring support for PostgreSQL, to achieve better performance by using asynchronous io. Now community already has similar attempt, for more details please refer https://anarazel.de/talks/2020-01-31-fosdem-aio/aio.pdf

# 预期成果/Expected Outcomes
- 通过 PostgreSQL 自带回归测试用例
- 通过 pgTAP 测试套
- 支持 io_uring 的 PostgreSQL 版本在 Aliyun Linux 2 上能正常运行
- 预期相比原始版本性能有 10%+ 的提升

- Pass PostgreSQL related testcases
- Pass pgTAP testcases
- Make PostgreSQL with io_uring support run normally on Aliyun Linux 2
- Expect 10+ performance optimization compared to original version

# 需要的技能/Required Skills
- 熟悉 C 语言编程
- 熟悉 PostgreSQL 的基本原理机制
- 熟悉异步 IO（aio/io_uring）

- Familiar with C
- Familliar with PostgreSQL
- Familliar with aio/io_uring

Start with the linked aio/io_uring reference and PostgreSQL's existing asynchronous I/O design; the issue does not identify specific source files or entry points. Review how the proposed change should integrate with PostgreSQL and define its test scope around PostgreSQL regression tests, pgTAP, Aliyun Linux 2 operation, and the stated performance target.

Written by the indexing model from the issue text.