futex_wait_private 时间长 6s 左右

**Describe the bug (描述bug)**
futex_wait_private 时间过长，straces看是因为 wait了 6s，才 wake up的。

**To Reproduce (复现方法)**
目前仅在自己开发机上复现，且无法解决

**Expected behavior (期望行为)**
期望不用wait这么久

**Versions (各种版本)**
OS:Linux
Compiler:gcc 4.8.5
brpc: 19年版本
protobuf:

**Additional context/screenshots (更多上下文/截图)**

Start with the provided strace showing futex_wait_private and review the Linux futex behavior in the context of brpc's 2019-era code and GCC 4.8.5 environment. Reproduce the roughly six-second wait outside the reporter's development machine if possible; done means identifying the cause and demonstrating that the unnecessary delay no longer occurs.

Written by the indexing model from the issue text.