CRM-19798 fix memory leak on EntityTag.get

CRM-19798 fix memory leak on EntityTag.get

This is an alternate methodology using __destruct on the DAO object.

Note that I have found some places bypass this - ie.

```
   $rule = new CRM_ACL_BAO_ACL();
   $rule->query($query);
```

But I think that is a pretty clumsy construct & we should swap to
CRM_Core_DAO::executeQuery();