Fix named locks implementation
Fix named locks implementation

* This patch fixes wrong usage of SQLAlchemy session in the
  implementation of named locks. See bug description for
  details.

Closes-Bug: #1727239
Change-Id: Ide40a6eebe056019e17b900f3f4c5acb94e7c24a

StackStorm/mistral,openstack/mistral,openstack/mistral,StackStorm/mistral