extindex|v2: defrag SQLite and Xapian DBs on btrfs

extindex|v2: defrag SQLite and Xapian DBs on btrfs

Doing periodic defrags ought to improve performance and perhaps
allow CoW to be usable with btrfs.  The autodefrag mount option
of btrfs(5) doesn't seem recommended by btrfs developers since
it's too aggressive, defragments too often, and wears out devices.
Performing defrag on our end should allow users to tune a more
ideal defrag interval to maintain performance while avoiding
excessive device wear.