Allow page lock to conflict among parallel group members.

Allow page lock to conflict among parallel group members.

This is required as it is no safer for two related processes to perform
clean up in gin indexes at a time than for unrelated processes to do the
same.  After acquiring page locks, we can acquire relation extension lock
but reverse never happens which means these will also not participate in
deadlock.  So, avoid checking wait edges from this lock.

Currently, the parallel mode is strictly read-only, but after this patch
we have the infrastructure to allow parallel inserts and parallel copy.

Author: Dilip Kumar, Amit Kapila
Reviewed-by: Amit Kapila, Kuntal Ghosh and Sawada Masahiko
Discussion: https://postgr.es/m/CAD21AoCmT3cFQUN4aVvzy5chw7DuzXrJCbrjTU05B+Ss=Gn1LA@mail.gmail.com