osd/SnapMapper: debug

Signed-off-by: Sage Weil <sage@inktank.com>
osd/SnapMapper: pass snaps set by const ref

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1698 from ceph/wip-snapmapper-debug

osd/SnapMapper: debug

Reviewed-by: Samuel Just <sam.just@inktank.com>