Allow specifying multiple UNIQUE constraints in the CREATE TABLE command,
or PRIMARY KEY and UNIQUE constraints, as long as they are all compatible
with the DISTRIBUTED BY clause.

Don't require the constraint's key columns to be a left-subset of the
DISTRIBUTED BY clause. It still needs to be a subset, but any subset will
do.

Likewise adjust the rules for deriving the distribution key from the
constraints, if no DISTRIBUTED BY was given, to choose the largest common
subset across all constraints.

Discussion: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/OJzz8I6WuVI/tMB8GcXdAgAJ