`.replace()` doesn't work for `time64` columns

It seems that `.replace()` doesn't have any effect when replacing times:

```python
from datatable import dt
from datetime import datetime as d

DT = dt.Frame([d(2000, 1, 1, 1, 1)])
DT.replace({d(2000, 1, 1, 1, 1) : d(2222, 1, 1, 1, 1)})
print(DT)
```

produces
```
| C0
| time64
-- + -------------------
0 | 2000-01-01T01:01:00
[1 row x 1 column]
```

should be
```
| C0
| time64
-- + -------------------
0 | 2222-01-01T01:01:00
[1 row x 1 column]
```