dump: Don't sort the output by message id.

dump: Don't sort the output by message id.

Asking xapian to sort the messages for us causes suboptimal IO patterns. This
would be useful, if we only wanted the first few results, but since we want
everything anyway, this is pessimization.

On 2011-10-29, a measurement on a 372981 messages instance showed that wall
time can be reduced from 28 minutes (sorted by Message-ID) to 15 minutes
(unsorted).

Timings on 189605 messages:

$ time notmuch.old dump
19.48user 5.83system 12:10.42elapsed 3%CPU (0avgtext+0avgdata 110656maxresident)k
3629584inputs+22720outputs (33major+7073minor)pagefaults 0swaps
$ echo 3 > /proc/sys/vm/drop_caches
$ time notmuch.new
14.89user 1.20system 3:23.58elapsed 7%CPU (0avgtext+0avgdata 46032maxresident)k
1256264inputs+22464outputs (43major+1990minor)pagefaults 0swaps