Hash password reset keys in the database.

All existing, unused password reset keys are now considered "expired" and the user will be told they should try again.

Introduces a password_reset_key_expired filter to allow plugins to introduce a grace period.

fixes #24783.

Built from https://develop.svn.wordpress.org/trunk@25696


git-svn-id: http://core.svn.wordpress.org/trunk@25611 1a063a9b-81f0-0310-95a4-ce76da25c4cd