Refactor the connection pool

[W0n9]

Found some issues in the staging environment:

1. Under high load, occasional deadlocks occur, causing the program to fail to respond normally, and user requests are backlogged in the buffer.
   
   Solution:
2. Introduce a new dynamically expandable connection pool, replacing the existing pre-filled connection pool, attempting to solving the deadlock problem existing in the old connection pool.
3. Introduce the testing of req.Body() to avoid req.Body() being cleared.
4. Upgrade the version of xerrors.

---

在预发环境中发现一些问题：

1. 高负载下偶发死锁，导致程序无法正常响应，用户请求积压在缓冲区
   

解决方案：

1. 引入新的动态扩展的连接池，代替现有的预填充的连接池，同时解决了现有连接池存在的死锁问题
2. 增加对req.Body()的测试，避免req.Body()被清空
3. 升级xerrors版本