Finish up the IMAP-only portion of iterative config reloading,
which allows us to create all sub-ranges of an inbox up front.
The InboxIdler still uses ->each_inbox which will struggle with
100K inboxes.
Having messages in the top-level newsgroup name of an inbox will
still waste bandwidth for clients which want to do full syncs
once there's a rollover to a new 50K range. So instead, make
every inbox accessible exclusively via 50K slices in the form of
"$NEWSGROUP.$UID_MIN-$UID_END".
This introduces the DummyInbox, which makes $NEWSGROUP
and every parent component a selectable, empty inbox.
This aids navigation with mutt and possibly other MUAs.
Finally, the xt/perf-imap-list maintainer test is broken, now,
so remove it. The grep perlfunc is already proven effective,
and we'll have separate tests for mocking out ~100k inboxes.