[threadpool] Split domain and worker management (#4117)

[threadpool] Split domain and worker management (#4117)

* [threadpool] Remove `-ms` suffix

* [threadpool] Split domain and worker management

This will allow us to use native threadpool more easily: we will simply have to implement a threadpool worker with the specific API.

On windows, we will use the Win32 Threadpool, and on OSX we will explore using GCD (Grand Central Dispatch).