User schema changes for latest tokenserver protocol

Supporting Firefox-Accounts-enabled Sync means we have to track a
"generation number" and "client state" for each user.  On the plus
side, we no longer need "conditions accepted" support.

See also:

  https://bugzilla.mozilla.org/show_bug.cgi?id=960009
  https://github.com/mozilla-services/tokenserver/pull/29