get_tez_session sometimes conflict because of thread unsafe regarding busy_sessions

When having multiple hue pages run tez applications at the same time, it, sometimes, will apply the same session to two different tasks, which will cause of of them receiving KILL signal and the other one complains that current app master is being used and retrying. I looked into the code of `HiveServerClient._get_tez_session` and I think the problem lies in the way `busy_sessions` is retrieved, which is not thread-safe. So there's chance that two query will be allocated to the same session when submitted virtually the same time.  I'd like to know is there any way to get current editor id (doc_id) from `HiveServerClient._get_tez_session` method, so I could do some hacking for a quick solution now. Thanks.