postgres: use stdlib to set serializable tx level
postgres: use stdlib to set serializable tx level

also use a context for the rollback, which is a bit cleaner since it
only results in one 'defer', rather than N from the loop

coreos/dex,coreos/dex,ericchiang/dex,ericchiang/dex,coreos/dex,wolfeidau/dex,ericchiang/dex,wolfeidau/dex,wolfeidau/dex