add logging to gitstatus lock/unlock
gitstatus.lock/unlock when running heavy operations

Like add_file, sync, etc.
rename tasks.gitstatus_update -> tasks.gitstatus_update_store

gitstatus_update will be a new task that updates status for a
single repo, which gitstatus_update_store will be the periodic
task for updating all of them.
added gitstatus_update task for updating individual repos async'ly
run gitstatus.update whenever a collection repo is modified

- Called with celery .apply_async when called from a view, so
  runs in the background.
- Called directly when run at the conclusion of a celery task.