Add a warning that iterating all sessions with redis can be very slow.

[jum]

I just did debug a problem that using Redis in a code path for an oauth2 flow was very slow compared to the version with the session store in a database. The problem turned out to be debugging code that used sessionstore.Iterate() to dump out all sessions which can be excruciatingly slow on a busy Redis server. In my case it took more than 10s to iterate through all sessions. So I thought it might be a good idea to warn that this might not be a good idea.