Add assert to ensure that page locks don't participate in deadlock cycle.

Add assert to ensure that page locks don't participate in deadlock cycle.

Assert that we don't acquire any other heavyweight lock while holding the
page lock except for relation extension.  However, these locks are never
taken in reverse order which implies that page locks will never
participate in the deadlock cycle.

Similar to relation extension, page locks are also held for a short
duration, so imposing such a restriction won't hurt.

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