Bug #2307: don't expire existing sessions when adding a secondary address

If a persona.org account is initially created with a "primary"
address (meaning an address served by a participating IdP, so
persona.org is given an assertion from that IdP as proof of ownership),
the new account will not have a password associated with it. If you then
add a "secondary" address (meaning an address *not* served by a
participating IdP, requiring an email challenge to prove ownership), you
will have to set up a password when you add the secondary. The
establishment of this password should *not* invalidate any sessions that
were set up earlier.

In Bug #2307, this manifested as the first browser (in which the
add-secondary-email operation was started, so it had the old session and
was waiting for the operation to complete, polling
/wsapi/email_addition_status all the while) receiving a "400
Unauthorized" error when the email challenge link was opened in a second
browser (which thus got a new session).

The test for this effect lives in tests/primary-then-secondary-test.js,
which need the same 2-second delay as password-update-test.js (to make
sure that the modified lastPasswordReset time was actually different
than the previous value, so the session really would be expired).