ObjectStore, *Store: extend fiemap interface

ObjectStore, *Store: extend fiemap interface

This extends fiemap interface in objectstore and all data stores so
it's possible to pass in the map used later by caller, instead of
encoding internal map into bufferlist, then parsing it back into the
same type of data container, wasting memory and CPU time in the process.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>