Merge branch 'revert_revert_issuable_lock' into 'master'

Revert the revert of Optimistic Locking

## What does this MR do?

It reverts the revert of optimistic lock https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5146 (and revert MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5245)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19871

Actually the code is already reviewed, except:
* Monkey patch `config/initializers/ar_monkey_patch.rb`
* We removed default values from migration `db/migrate/20160707104333_add_lock_to_issuables.rb`

See merge request !5623