tag popluation should be done in a post-commit hook

Today, when a tag is created (or modified), the work of populating the tag across existing nodes is farmed out to the rack controllers, who then run through and do the actual work. If the rack controller manages to find a node which should have the tag before the region finishes committing the tag to the database, then the node update will fail.

The solution is to always populate_tags from a post-commit hook on Tag.save(), rather than directly from the save() function.