conformance: align with upstream conformance tests

Upstream conformance tests are being updated, so we need to align along
with our internal GC and dedupe features.

Add a new example config file which plays nice with conformance tests.

DeleteImageManifest() updated to deal with the case where the same
manifest can be created with multiple tags and deleted with the same
digest - so all entries must be deleted.

DeleteBlob() delete the digest key (bucket) when last reference is
dropped

pkg/api/routes.go

@@ -222,7 +222,12 @@ func (rh *RouteHandler) ListTags(w http.ResponseWriter, r *http.Request) {
 			pTags.Tags = tags[i+1 : i+1+n]
 		}
 
-		last = pTags.Tags[len(pTags.Tags)-1]
+		if len(pTags.Tags) == 0 {
+			last = ""
+		} else {
+			last = pTags.Tags[len(pTags.Tags)-1]
+		}
+
 		w.Header().Set("Link", fmt.Sprintf("/v2/%s/tags/list?n=%d&last=%s; rel=\"next\"", name, n, last))
 		WriteJSON(w, http.StatusOK, pTags)