Chert backend

[Sebastian Spaeth]

Hi all, just a reminder for all those who were annoyed with notmuch
performance.

I was still using the xapian in my ubuntu (1.0.20) which did not offer
the chert backend. I have now installed the newer libxapian (can exist
in parallel) and libxapian-dev and have converted my notmuch store to
chert rather than the flint backend.

It boils down to remove the 'xapian' dir and recreate it with:
XAPIAN_PREFER_CHERT=1 notmuch new
(don't forget to dump and restore your tags :-)).

The performance difference on my encrypted home partition is night and
day for me. I am not sure if the old db had been collecting cruft or
whatnot, but things are much speedier using chert.

So if you have an "imamflint" file in your xapian directory you might
want to try the same on your distro.

Sebastian