[Refactor] Use async scheduler service methods instead of jupyterlab commands

[andrii-i]

This really should be async as well, but it looks like we're calling stop job via JL commands instead of simply awaiting the scheduler service methods. Refactoring those is outside the scope of this PR.

Originally posted by @dlqqq in https://github.com/jupyter-server/jupyter-scheduler/pull/245#discussion_r1010742610