To allow inserts in parallel-mode this feature has to ensure that all the
constraints, triggers, etc. are parallel-safe for the partition hierarchy
which is costly and we need to find a better way to do that. Additionally,
we could have used existing cached information in some cases like indexes,
domains, etc. to determine the parallel-safety.

List of commits reverted, in reverse chronological order:

ed62d373 Doc: Update description for parallel insert reloption.
c8f78b61 Add a new GUC and a reloption to enable inserts in parallel-mode.
c5be48f0 Improve FK trigger parallel-safety check added by 05c8482f.
e2cda3c2 Fix use of relcache TriggerDesc field introduced by commit 05c8482f.
e4e87a32 Fix valgrind issue in commit 05c8482f.
05c8482f Enable parallel SELECT for "INSERT INTO ... SELECT ...".

Discussion: https://postgr.es/m/E1lMiB9-0001c3-SY@gemulon.postgresql.org