vfs: take advantage of bup chunking to make file seeking faster.

vfs: take advantage of bup chunking to make file seeking faster.

If you have a huge file, you can now seek around inside it (eg. in 'bup
fuse') without having to read its entire contents.  Calculating the file
size is also really fast now.

This makes a bup fuse-mounted filesystem much more useful for real-time
access.  For example, I was able to connect to an sqlite3 database and have
it work at a reasonable speed.  (Obviously, since 'bup fuse' is written in
python and doesn't currently support threading, the speed could still be
improved, but at least it's no longer godawful.)