ReplicatedPG:: s/_delete_head/_delete_oid, adjust head_exists iff is_head

Fixes: #7872
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1554 from ceph/wip-7828

ReplicatedPG:: s/_delete_head/_delete_oid, adjust head_exists iff is_hea...

Reviewed-by: Sage Weil <sage@inktank.com>