Fix race condition in refreshing "join" task state
Fix race condition in refreshing "join" task state

* Previously we used periodic jobs to refresh state of "join" tasks
  and there was a guarantee that only one such job could run at a
  time, so there wasn't a need in using locking. Now we allow more
  than one such jobs run in parallel processes (and threads) so
  we have to lock task execution and then check task state again
  and update it, if needed.

Change-Id: Icaad486d9c3f830db0314dedb44664940cca0014
Closes-Bug: #1803483

openstack/mistral,openstack/mistral