[#360] Use 'c' prefix for bucket policies instead of 'n'

With 'c' prefix, acl chains become shorter, thus gateway
receives shorter results and avoids sessions to neo-go.

There is still issue with many IAM rules.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

api/router_test.go

@@ -197,6 +197,7 @@ func TestPolicyChecker(t *testing.T) {
 func TestPolicyCheckerReqTypeDetermination(t *testing.T) {
 	chiRouter := prepareRouter(t)
 	bktName, objName := "bucket", "object"
+	createBucket(chiRouter, "", bktName)
 
 	policy := engineiam.Policy{
 		Version: "2012-10-17",