Add debug log to indicate when external lock is taken
Add debug log to indicate when external lock is taken

In a case when a number of threads are concurrently accessing a
resource that has a external lock there is no indication when
one of the threads has taken the external lock.

This patch adds in a debug log to try and add some clarity.

Change-Id: Ibd7cfd3b39370b1a54f2ab50c75c6b92a876ae1a

openstack/oslo.concurrency