Clean up schema for table lfs_objects

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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: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Missing constraint: file NOT NULL
• Wrong data type: oid should be bytea - save 70 MB
• Wrong data type: file should be bytea - save 70 MB
• Name change: "oid" is used in PG catalogs, recommend changing to another name
• Duplicate data: Could 'id' be used instead of 'oid', removing the need for oid entirely? (save 92MB)