caos.backends.pgsql: Fix creation of inherited pointers with differing defaults
caos.backends.pgsql: Fix creation of inherited pointers with differing defaults

Considering the following schema:

   A:
       links:
           link1: {bool: {default: False}}

   B:
       extends: A
       links:
           link1: {bool: {default: True}}

In cases when both links were created at the same time, delta backend
would fail to apply the differing default in B.link1 to the database
schema.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb