Refactor the logic for setting to memcached.

First we've renamed the oldSessionUser variable to sessionUser, as that
is most accurate in what we're actually doing. The logic implemented is
as follows:

  - If there's no user on the session - don't save!
  - If there *is* a user on the session, and the data is the same as what
    we have in memcache already - don't save!
  - For everything else - save it.