API/BUG: return np.nan rather than -1 for invalid datetime accessors values (GH8689)
BUG: millisecond Timestamp/DatetimeIndex accessor fixed

closes #8689 

@jorisvandenbossche note that this is a slight API change, but fixes a long-standing oddity (the .milliseconds).
 @jorisvandenbossche I removed the millisecond stuff...still save for later 
 yep, good to split it. The `-1/nan` thing can certainly be regarded as a bug
