With this change, existing session cookies are invalidated when the user
changes their password. This will also support a "log me out of
everywhere" button, which merely needs to update the superSessionToken
as if the password was changed.

This adds .superSessionToken to all sessions in
wsapi.authenticateSession, and checks it against the database row in
during a new checkExpiredSession() call (run for all /wsapi requests,
both GETs and POSTs).

All database methods which create an account or modify the password were
changed to also set .superSessionToken (to the current time).
updatePassword() added an 'invalidateSessions' argument so passwords can
be rehashed (when the bcrypt work factor changes) *without* invalidating
sessions. Finally, the database added a new method named
superSessionToken() to retrieve the current value, for comparison in
checkExpiredSession().

wsapi/update_password.js is changed to add a call to authenticateSession
after changing the password, to update the cookie with the new
.superSessionToken. This ensures that the user's new session works on
the next call after update_password.

Database rows that don't have a superSessionToken (created before the
code was upgraded to include this change) will not enforce
session-checking, so user sessions will not be spontaneously expired
when the server is upgraded past this revision.