osd: create tool to extract pg info and pg log from filestore

New application ceph-filestore-dump created that mounts filstore
and can dump info or log in JSON when an OSD is not running.

Feature: #3890

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>