Avoid unnecessary shm writes in Parallel Hash Join.

Avoid unnecessary shm writes in Parallel Hash Join.

Currently, Parallel Hash Join cannot be used for full/right joins,
so there is no point in setting the match flag.  It turns out that
the cache coherence traffic generated by those writes slows down
large systems running many-core joins, so let's stop doing that.
In future, if we need to use match bits in parallel joins, we might
want to consider setting them only if not already set.

Back-patch to 11, where Parallel Hash Join arrived.

Reported-by: Deng, Gang
Discussion: https://postgr.es/m/0F44E799048C4849BAE4B91012DB910462E9897A%40SHSMSX103.ccr.corp.intel.com