[API Server] `sky api cancel` should be execute immediately and should not go into request queue

[cblmemo]

Currently, sky.api_cancel goes to request queue and when there are lots of ongoing request, it is really hard to cancel them.

Cancel requests should be able to interrupt other on-going requests. It was probably because the api cancel is too costly in our implementation, so we left it in a worker to schedule it on a separate process. We should try to make it immediate.