improved job API

* added REST API method HTTP GET `/_api/job/job-id` to query the status of an
  async job without potentially fetching it from the list of done jobs

* fixed non-intuitive behaviour in jobs API: previously, querying the status
  of an async job via the API HTTP PUT `/_api/job/job-id` removed a currently
  executing async job from the list of queryable jobs on the server.
  Now, when querying the result of an async job that is still executing,
  the job is kept in the list of queryable jobs so its result can be fetched
  by a subsequent request.