Close site too slow with many open login sessions

[Richard Mansfield]

Submitting the 'close site' button in the admin section tries to close all the open sessions, one by one, using session_destroy(). Turns out this is okay for suspending a single user, but it wasn't a great idea when you're closing the site & need to wipe out lots of sessions.

Maybe we should change it to just delete the session files from disk; it'd be faster. Don't know enough about php sessions to be sure whether it'd work; I guess it's possible they're just cached somewhere and get written out again periodically.