Clean up schema for table path_locks

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing constraint: project_id NOT NULL
• Missing constraint: user_id NOT NULL
• Missing constraint: UNIQUE (path, project_id, user_id) (3 cases where this rule is violated), the user_id here makes no sense