Looks like this was just a bug in Xapian HEAD, today's build works just
fine and I'm enjoying the faster CHERT backend.  Bulk tagging is really
loads faster now, and the interactivity is better than before, but SPC
still has a noticeable pause (at least 200 ms) since (apparently) it's
doing an fsync, and this is worse when the disk is getting hit by some
other job (like when I'm reading email while something big compiles).
It wouldn't bother me at all if I lost my last few seconds of
interactive tagging due to something catastrophic like losing power.  I
think there is still (post #250) a case for supporting some asynchronous
operations.

Jed