Report a special error code when assertion has a bad timestamp.
Change backend interface and schema for latest protocol changes.

To support Firefox-Accounts-enabled sync, we need to track a
"generation number" and a "client state" for each user, and to
re-assign their uid when the client state changes.  This adds
the necessary interface changes to support that.

On the plus side, we no longer need conditions-accepted so that
has been removed in the shuffle.
Change "Browser-ID" to "BrowserID" to match upstream terminology.
Never try to update to an old client-state string.
Fix review nits.
Merge pull request #29 from mozilla-services/rfk/backend-record-changes

Bug 960009 - implement latest tokenserver protocol changes; r=telliott