Update WIMMS for new tokenserver user schema.

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.
Add a limit to the size of 'old_client_states' fetched from the db.

This shouldn't affect good clients, but may prevent badly-behaved
clients from swamping us with nonsense records to be held in memory.
Throw a BackendError if attempt to use an old client-state string.
Fix review nits and flake.
Merge pull request #3 from mozilla-services/rfk/user-schema-changes

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