replace user save with lookup on duplicate data errors

[ericcj]

it's possible for user saves to be retried on timeouts but have actually succeeded in creating the user, in which case we raise

HTTP Status 400 Body {"code":202,"error":"username miUnNsGPHt already taken"}

but we should be looking up the user that was created on the previous retry in those cases so as not to have the same bug as https://parse.com/questions/username-already-taken-error-on-signup-retry