Transferring projects is performed synchronously

Summary

Project transfers are performed synchronously in Unicorn, rather than being backgrounded in sidekiq. Since this involves a number of mv commands, as well as database updates, this scales poorly and is prone to errors. This particularly came to light via https://gitlab.com/gitlab-com/support-forum/issues/2036

Of particular note is that a large number of mv commands happen while holding open a database transaction.

Steps to reproduce

Transfer a project between namespaces

What is the current bug behavior?

File moves done in unicorn

What is the expected correct behavior?

File moves done in sidekiq

Possible fixes

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/projects_controller.rb#L68

Wrap this (and the equivalent admin action) in a ProjectTransferWorker.

Alternatively, we could do the database synchronously and just enqueue the file moves asynchronously.

It's worth noting that the rollback logic in this service is incomplete, too.