[Format] Days since epoch date type

[asfimport]

While we've decided to make the primary IPC date type be int64 milliseconds since the UNIX epoch, in many libraries dates are represented as integer (int32, usually) days since some epoch. In the Python standard library, the epoch is the year 0:

>>> d = datetime.date(2017, 1, 17)
>>> d.toordinal()
736346
>>> d.toordinal() / 365
2017

At least in Cplusplus-land, in working on ARROW-452 I ran into the problem of how to do zero-copy reads of such data, while preserving the metadata to know that the values are dates. I added a cpp-only "date32" type to support this use case #365

I'm not sure whether we should add a new logical type, but thought it would be worth bringing up in any case

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Finalize Date type (is related to)

_{Note: This issue was originally created as ARROW-608. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
@julienledem I linked with ARROW-316

[asfimport]

Wes McKinney / @wesm:
Done in 98a52b4#diff-1ddcb92ade31c8fbd370001f9b29a7d9