feat(scheduler): add graceful stop method to scheduler

[Rilord]

No description provided.

[reugn]

@Rilord, thanks for your interest in the project. Could you elaborate on the motivation behind adding this new method to the scheduler? The existing 'Stop and then Wait' functionality might already cover the use case you're trying to address.

[Rilord]

@reugn The motivation behind this PR is to provide method which gracefully shutdowns whole scheduler without stopping/killing running jobs, i.e. shutdown it without killing the scheduler's context. You can actually achieve almost the same result by using Clear and then Wait.