InboxIdle should not be holding onto Inbox objects after the
Config object they came from expires, and Config objects may
expire on SIGHUP.
Old Inbox objects still persist due to IMAP clients holding onto
them, but that's a concern we'll deal with at another time, or
not at all, since all clients expire, eventually.
Regardless, stale inotify watch descriptors should not be left
hanging after SIGHUP refreshes.