Revert "Merge branch '207312-remove-optimisticlocking-monkeypatch' into 'master'"

...nreleased/207312-remove-optimisticlocking-monkeypatch.yml

...initializers/config_initializers_active_record_locking.rb

...e_previous_schema_without_lock_version_null_constraint.rb

...4723_restore_previous_schema_with_lock_version_indices.rb

...st_migrate/20200608205813_set_lock_version_to_not_null.rb

..._migrate/20200608212030_lock_version_cleanup_for_epics.rb

...20200608212435_lock_version_cleanup_for_merge_requests.rb

...migrate/20200608212549_lock_version_cleanup_for_issues.rb

...rate/20200608212652_lock_version_cleanup_for_ci_stages.rb

...rate/20200608212807_lock_version_cleanup_for_ci_builds.rb

db/structure.sql

...ations/cleanup_optimistic_locking_nulls_pt2_fixed_spec.rb

spec/models/issue_spec.rb

spec/models/merge_request_spec.rb