ceph-objectstore-tool: add a way to split filestore directories offline

ceph-objectstore-tool: add a way to split filestore directories offline

Use the usual split code, and split each dir that meets the
usual split criteria.

This can be run with lower than usual split settings, to avoid costly
online splits. To make sure the directories are not merged again, use
a load merge threshold (e.g. filestore merge threshold = 1), and
adjust the split multiplier accordingly.

Fixes: http://tracker.ceph.com/issues/17220
Signed-off-by: Josh Durgin <jdurgin@redhat.com>