This reuses existing InboxIdle infrastructure to update external
indices based on per-inbox updates. This is an alternative to
auto-updating external indices via the -index command and also
works with existing uses of -mda and public-inbox-watch.
Using inotify (or EVFILT_VNODE) allows watching thousands of
inboxes without having to scan every single one at every
invocation.
This is especially beneficial in cases where an external index
is not writable to the users writing to per-inbox indices.