IntervalArray.shift and missing value handling
Followup to https://github.com/pandas-dev/pandas/pull/22387

The default implementation of `shift` fails when `dtype.na_dtype` can't be stored in a `dtype` array (e.g. int can't hold na).

```python
In [24]: idx = IntervalArray.from_breaks(range(10))

In [25]: idx.shift()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-25-1b2c2192e1e6> in <module>()
----> 1 idx.shift()

~/sandbox/pandas/pandas/core/arrays/base.py in shift(self, periods)
    422             return self.copy()
    423         empty = self._from_sequence([self.dtype.na_value] * abs(periods),
--> 424                                     dtype=self.dtype)
    425         if periods > 0:
    426             a = empty

~/sandbox/pandas/pandas/core/arrays/interval.py in _from_sequence(cls, scalars, dtype, copy)
    193     @classmethod
    194     def _from_sequence(cls, scalars, dtype=None, copy=False):
--> 195         return cls(scalars, dtype=dtype, copy=copy)
    196
    197     @classmethod

~/sandbox/pandas/pandas/core/arrays/interval.py in __new__(cls, data, closed, dtype, copy, fastpath, verify_integrity)
    138
    139         return cls._simple_new(left, right, closed, copy=copy, dtype=dtype,
--> 140                                verify_integrity=verify_integrity)
    141
    142     @classmethod

~/sandbox/pandas/pandas/core/arrays/interval.py in _simple_new(cls, left, right, closed, copy, dtype, verify_integrity)
    156                 raise TypeError(msg.format(dtype=dtype))
    157             elif dtype.subtype is not None:
--> 158                 left = left.astype(dtype.subtype)
    159                 right = right.astype(dtype.subtype)
    160

~/sandbox/pandas/pandas/core/indexes/numeric.py in astype(self, dtype, copy)
    316         elif is_integer_dtype(dtype) and self.hasnans:
    317             # GH 13149
--> 318             raise ValueError('Cannot convert NA to integer')
    319         return super(Float64Index, self).astype(dtype, copy=copy)
    320

ValueError: Cannot convert NA to integer
```

Perhaps we can investigate using our IntegerNA extension array for the storage of int-dtyped IntervalArrays?