The newly introduced `Scheduler::GetSequenced()` returns
sequenced schedulers from the cache limited to 10 instances,
preventing from spawning too many threads.
This commit refactors `utils::ThreadPool` into a template
`ThreadedScheduler` class and provides aux type aliases.
So that it is possible to obtain a sequenced schedule,where
all the scheduled tasks are guarantied to be executed
consequently.
The sequenced lightweight scheduler is required by both the
orchestration thread and the refactored `FileSource` implementation.
- Do not carry it over everywhere as parameter, it is a shared
instance anyway and the lifecycle is pretty much the app lifecycle
from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
do tasks in the background, we don't make assumptions if it is a
thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.
