Makefile: add SnapMapper to osd

Signed-off-by: Samuel Just <sam.just@inktank.com>
SnapMapper: add remove_oid and get_snaps

Signed-off-by: Samuel Just <sam.just@inktank.com>
test_snap_mapper: add tests for remove_oid and check_oid

Signed-off-by: Samuel Just <sam.just@inktank.com>