Make nested groups deletion synchronous to avoid potential race

There may be a potential race condition in the group deletion. Suppose there is a namespace test with nested group foo:

1. Namespace test is queued for deletion
2. Namespace test schedules foo for deletion
3. Namespace test is destroyed
4. Namespace foo finds no parent, so it may fail

Didn't have time to write a spec for this, but I wanted to raise this as a potential issue.

changed milestone to %9.0

@stanhu makes total sense, thanks

added 870 commits

• bac125d4...8b855eaf - 868 commits from branch master
• 3129dd50 - Make nested groups deletion synchronous to avoid potential race
• 334591dd - Include nested group into destroy_service_spec

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unmarked as a Work In Progress

enabled an automatic merge when the pipeline for 334591dd succeeds

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/2772

merged