Clean up schema for table merge_requests
From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- Wrong data type: locked_at should be timestamptz
- Wrong data type: deleted_at should be timestamptz
- Wrong data type: merge_commit_sha should be bytea
- Wrong data type: rebase_commit_sha should be bytea
- Wrong data type: in_progress_merge_commit_sha should be bytea
- Missing FK: author_id -> users (14522 entries violate this rule)
- Missing FK: assignee_id -> users (1962 entries violate this rule)
- Missing FK: updated_by_id -> users (602 entries violate this rule)
- Missing FK: merge_user_id -> users (135 entries violate this rule)
- Missing FK: source_project_id -> projects
- Missing FK: target_project_id -> projects
- Missing FK: milestone_id -> milestones
- Duplicate data: merge_status would be more efficient as an enum or integer (or smaller), save ~34MB