Rewrite SubscriptionManager to async framework (Version: [2.1.0])

[godexsoft]

Summary

SubscriptionManager still uses std::thread.
Looks like we don't need ioContext_ in SubscriptionManager itself.

Solution

Rewrite std::thread to async framework.
Also delete unused ioContext_.

Paths Not Taken

Keep std::thread in codebase

[godexsoft]

Subscription manager was rewritten and uses io_context on a number of threads. This can be rewritten using AsyncFramework.