Don't explicitly mark range partitioning columns NOT NULL.

Don't explicitly mark range partitioning columns NOT NULL.

This seemed like a good idea originally because there's no way to mark
a range partition as accepting NULL, but that now seems more like a
current limitation than something we want to lock down for all time.
For example, there's a proposal to add the notion of a default
partition which accepts all rows not otherwise routed, which directly
conflicts with the idea that a range-partitioned table should never
allow nulls anywhere.  So let's change this while we still can, by
putting the NOT NULL test into the partition constraint instead of
changing the column properties.

Amit Langote and Robert Haas, reviewed by Amit Kapila

Discussion: http://postgr.es/m/8e2dd63d-c6fb-bb74-3c2b-ed6d63629c9d@lab.ntt.co.jp