bthread_concurrency 支持减少 concurrency

**Is your feature request related to a problem?**
N/A

**Describe the solution you'd like**
希望 bthread_concurrency 可以支持减少 concurrency

**Describe alternatives you've considered**
我们使用 Bthread 实现了协程池，同时我们使用 Bthread Tag Group 来实现不同协程池之间底层 Worker 的分组。现在我们期望不同协程池的 Worker 数量可以动态调节，以灵活适应不同的场景。
但是我们翻阅代码发现，bthread_concurrency 和 bthread_concurrency_by_tag 两个均不支持减少 concurrency。
请问这里不支持减少 concurrency 是否存在什么考量？以及是否有好的建议帮助我们实现这样的功能呢？感谢！

**Additional context/screenshots**
N/A

No file or test is named. Begin by locating the implementations and call sites for bthread_concurrency and bthread_concurrency_by_tag, then define how reducing concurrency should affect existing workers and tagged pools. Done should include dynamic reductions for both APIs with coverage for the requested worker-pool behavior.

Written by the indexing model from the issue text.