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>
(cherry picked from commit e7b0428e0e8d8f5459311dc698d94a3ac4f04684)

Conflicts:
	src/tools/ceph_objectstore_tool.cc
* remove c++11 auto usage
* change HashIndex::list_subdirs() call to use set instead of vector
* adjust to hammer signature of coll_t::is_pg()