ceph_filestore_dump: Export omap in batches for large omap case

New function get_omap_batch()
Create a TYPE_OMAP section for each batch

Signed-off-by: David Zafman <david.zafman@inktank.com>
Remove unused arguments to do_list_omap() and do_list_attrs()
Fix do_list_omap() to use get_oamp_batch() to operate in batches
Improve usage messages and tests