Fix bug where projects would appear to be stuck in the forked import state

A race condition existed between when Rails committed the import_status to started and when the Sidekiq worker forked a project. If this fork were quick, it's possible that the worker would attempt to move into the finished state before the started state took effect.

As mentioned in https://github.com/mperham/sidekiq/wiki/Problems-and-Troubleshooting#cannot-find-modelname-with-id12345, we can either delay the worker to ensure the DB has a chance to update, or use the nice after_commit_queue gem to schedule the task after the state machine commit. See:

• https://github.com/pluginaweek/state_machine/issues/191
• https://github.com/shellycloud/after_commit_queue

Closes #2736 (closed)