Orphan content_lock entries ofdeleted content

We've noticed a very hard to reproduce bug with content lock due to the nature of our development and testing environments here at the City of Portland.

It seems that when content that is locked is deleted, the content's corresponding entry in the content_lock table remains. This would normally not be a problem, as the nid of any future node would auto increment, but we have a set up that relies on backups from multiple environments the details of which aren't really all that important to the issue, but the short story is that we frequently get hit with brand new content in our local development environments being locked as a result of our behat tests that run against our test environments.

We think that the content_lock module should augment or implement a hook for content deletion that will clean up any potentially orphaned content_lock table entries to prevent the issue from happening.

Steps to reproduce:

1. Create a new content node
2. Lock the content node by editing
3. Verify that the content_lock table has an entry for that node's nid
4. Delete the node without first breaking the lock
5. Verify that the content_lock entry for that node remains