REF: share join methods for DTI/TDI
The only actual changed behavior is in the new `_is_convertible_to_index_for_join` method where it first checks:

```
if isinstance(other, cls):
    return False
```

In master, TimedeltaIndex uses _is_convertible_to_index which returns True here.  The DTI method behavior is unchanged.
In a follow-up, there will be more set methods we can share. ping