Navigating away from a node removes the lock but not the locked message.

Problem/Motivation

Navigating away from a node removes the lock but not the locked message.

Steps to reproduce:
1. Start editing a node to create a lock on that node.
2. Navigate away by clicking the "back" button.
3. Click the "leave page" button on the dialog that asks you if you want to leave the node you're editing.
4. Once back to the view of the node, click the browser refresh button
5. The node is locked message is displayed, even though the lock has been deleted.
6. Click refresh again, the message disapears.

Actual results:
After navigating away from a node being edited, the node is locked message is displayed once, even though the lock has been deleted.

Expected results:
You should never see the "Node is locked" message if the lock has already been deleted.

Proposed resolution

Anytime the a lock is deleted, any pending messages in $_SESSION['content_lock'] for that nid should also be deleted.

Remaining tasks

Apply attached patch.