librbd: reimplement striping

librbd: reimplement striping

This replaces most of the existing striping code with use of the Filer
striping helper methods and a more general ceph_file_layout that can
handle more sophisticated striping patterns that the previous uniform
object approach.

This patch is not fully complete; there are a few additional patches that
follow that clean up some of the support functions.  However, most of the
IO path is covered here.

Signed-off-by: Sage Weil <sage@inktank.com>