osd: require SNAPMAPPER feature from peers

This was introduced before cuttlefish.  We require users to upgrade first
to a newer release, so there is no need to support a mixed cluster with
such old code.

Signed-off-by: Sage Weil <sage@redhat.com>
os/ObjectStore: drop tolerate_collection_add_enoent

This is no longer needed and clutters the interface.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2997 from ceph/wip-snapmapper

osd: require SNAPMAPPER feature; drop tolerate_collection_add_enoent

Reviewed-by: Samuel Just <sjust@redhat.com>