librbd: enable rbd cache by default; writethrough until flush

This is recommended and safe.  Avoid some user confusion.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2123 from ceph/wip-rbd-flush

librbd: enable rbd cache by default; writethrough until flush

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>