ROADMAP: Consistent missing value handling with new NA scalar
I cleaned up my initial write up on the consistent missing values proposal (https://github.com/pandas-dev/pandas/issues/27825#issuecomment-520583911), and incorporated the items brought up in the last video chat. So I think it is ready for some more detailed discussion.

The last version of the full proposal can be found here: https://hackmd.io/@jorisvandenbossche/Sk0wMeAmB

TL;DR:

- I propose to introduce a new scalar (singleton) `pd.NA` that can be used as the missing value indicator (when accessing a single value, not necessarily how it is stored under the hood). 
- This can be used instead of `np.nan` or `pd.NaT` in new data types (eg nullable integers, potential string dtype)
- Long term, we can see if there is a migration possible to use this consistently for all data types.

cc @pandas-dev/pandas-core 