线程模型支持协程

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当前线程模型支持一个连接一个线程和线程池模式，我们可以扩展更多的线程模型。协程可以让程序运行的更加高效，已经有比较多的开源库支持了协程开发，并且较新的C++标准也对协程做了支持。

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引入第三方协程库或者依赖新的C++标准实现自己的协程库。

第三方库比如 boost、libco、libgo等。

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协程对应用层程序开发和库本身都有较高的要求，一些耗时的系统调用，比如 connect、read、write 等都可以在协程库实现hook，但是像mutex等，如果实现不好或者使用不当，会导致系统很不稳定。

No files, tests, or entry points are named. Start by locating the existing one-connection-per-thread and thread-pool implementations, then review the project's C++ standard and dependency configuration. Define the coroutine library or C++ standard approach and the behavior of blocking calls such as connect, read, and write before implementation can be considered complete.