added better error handling. Also refactored some of the code and fixed a few issues in project_tree_saver

Run ProjectDestroyWorker after pending_delete attribute has been committed to DB

Closes gitlab-org/gitlab-ee#506

This allows us to track how much time is spent in updating the
"pushes_since_gc" column as well as the time needed to obtain the lease.

GitHub Pull Requests importer handle with the repository while
importing data, we need to make sure that the cached values are valid.