There was some confusion on how NULLs are distributed, when CdbPathLocus
is of Hashed or HashedOJ type. The comment in cdbpathlocus.h suggested
that NULLs can be on any segment. But the rest of the code assumed that
that's true only for HashedOJ, and that for Hashed, all NULLs are stored
on a particular segment. There was a comment in cdbgroup.c that said "Or
would HashedOJ ok, too?"; the answer to that is "No!". Given the comment
in cdbpathlocus.h, I'm not suprised that the author was not very sure
about that. Clarify the comments in cdbpathlocus.h and cdbgroup.c on that.

There were a few cases where we got that actively wrong. repartitionPlan()
function is used to inject a Redistribute Motion into queries used for
CREATE TABLE AS and INSERT, if the "current" locus didn't match the target
table's policy. It did not check for HashedOJ. Because of that, if the
query contained FULL JOINs, NULL values might end up on all segments. Code
elsewhere, particularly in cdbgroup.c, assumes that all NULLs in a table
are stored on a single segment, identified by the cdbhash value of a NULL
datum. Fix that, by adding a check for HashedOJ in repartitionPlan(), and
forcing a Redistribute Motion.

CREATE TABLE AS had a similar problem, in the code to decide which
distribution key to use, if the user didn't specify DISTRIBUTED BY
explicitly. The default behaviour is to choose a distribution key that
matches the distribution of the query, so that we can avoid adding an
extra Redistribute Motion. After fixing repartitionPlan, there was no
correctness problem, but if we chose the key based on a HashedOJ locus,
there is no performance benefit because we'd need a Redistribute Motion
anyway. So modify the code that chooses the CTAS distribution key to
ignore HashedOJ.

While we're at it, refactor the code to choose the CTAS distribution key,
by moving it to a separate function. It had become ridiculously deeply
indented.

Fixes https://github.com/greenplum-db/gpdb/issues/6154, and adds tests.
Reviewed-by: NMelanie Plageman <mplageman@pivotal.io>