cdef out dtype for _Timestamp._get_field
Follow-up to @jreback [comment](https://github.com/pandas-dev/pandas/pull/17377#discussion_r136948947) in #17377 to dtype the `out` variable in `_get_field` and `get_named_field`

Also, replaced value conversion in `_get_start_end_field` with `_maybe_convert_value_to_local` function created in #17377