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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master

Details
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https://git.postgresql.org/pg/commitdiff/cc32ec24fdf3b9808617439d41b0ba7f3eb16b04

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(-)