request in handle_to_request_map will not clear if backgroupd is True

[jiangrzh]

when client submitted a background job, it seems woker will not send back any state except 'CREATE', and therefore the request in handle_to_request_map will not pop any time:
‘’‘python
def recv_job_created(self, job_handle):
if not self.requests_awaiting_handles:
raise InvalidClientState('Received a job_handle with no pending requests')

    # If our client got a JOB_CREATED, our request now has a server handle
    current_request = self.requests_awaiting_handles.popleft()
    self._assert_request_state(current_request, JOB_PENDING)

    # Update the state of this request
    current_request.job.handle = job_handle
    current_request.state = JOB_CREATED

    #  The question is: when to unregister the request if background was True? 
    self._register_request(current_request)      

    return True

’‘’