tools/ceph-filestore-dump: Implement remove, export and import

Change local names to be clearer
Break real_log() into common function get_log()
Move infos_oid, biginfo_oid and log_oid to globals for general use

Feature: #4201 (osd: data loss: pg export/import/remove)

Signed-off-by: David Zafman <david.zafman@inktank.com>