Time-limit retry attempts

[danfinlay]

At least for the public launch, we should have a background-enforced time-limiting on password retries, to reduce plausibility of brute forcing a metamask vault. Maybe the retry interval should increase logarithmically, too.

[kumavis]

this should be done at the crypto level by deriving the privatekey via a time-intensive hash bc if you have access to the computer you can just grab the encrypted lightwallet data from localstorage and brute force it back in your evil crypto lair