inbox: drop memoization/preload, cleanup expires caches
cloneurl, description, and base_url are no longer memoized. The
non-$env form of base_url is rare in WWW, and is fast enough to
not require memoization.
cloneurl and description are now expired during cleanup,
allowing admins to change these files without restarting
(or SIGHUP).
-altid_map is no longer cached nor memoized at all, since the
endpoint(s) which hit it seem rarely accessed.
nntp_url and imap_url are now cached (instead of memoized) in
case an inbox is unvisited for a long time. They remain cached
since the truthiness check gets called in every per-inbox HTML
page, which can potentially be expensive.