pgsql: Revert the commits related to allowing page lock to conflict amo

Revert the commits related to allowing page lock to conflict among parallel group members.

This commit reverts the work done by commits 3ba59ccc89 and 72e78d831a.
Those commits were incorrect in asserting that we never acquire any other
heavy-weight lock after acquring page lock other than relation extension
lock. We can acquire a lock on catalogs while doing catalog look up after
acquring page lock.

This won't impact any existing feature but we need to think some other way
to achieve this before parallelizing other write operations or even
improving the parallelism in vacuum (like allowing multiple workers
for an index).

Reported-by: Jaime Casanova
Author: Amit Kapila
Backpatch-through: 13
Discussion: https://postgr.es/m/CAJKUy5jffnRKNvRHKQ0LynRb0RJC-o4P8Ku3x9vGAVLwDBWumQ@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3c1adbbf86c2cfa44ebed64bd01ed589ad0b832b

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 12 +++++++-----
src/backend/storage/lmgr/README      | 34 +++++++++++++---------------------
src/backend/storage/lmgr/deadlock.c  |  9 ++++-----
src/backend/storage/lmgr/lock.c      | 32 ++++----------------------------
src/backend/storage/lmgr/proc.c      | 12 ++++++------
5 files changed, 34 insertions(+), 65 deletions(-)