contrib/isn: Make weak mode a GUC setting, and fix related functions.

contrib/isn: Make weak mode a GUC setting, and fix related functions.

isn's weak mode used to be a simple static variable, settable only
via the isn_weak(boolean) function.  This wasn't optimal, as this
means it doesn't respect transactions nor respond to RESET ALL.

This patch makes isn.weak a GUC parameter instead, so that
it acts like any other user-settable parameter.

The isn_weak() functions are retained for backwards compatibility.
But we must fix their volatility markings: they were marked IMMUTABLE
which is surely incorrect, and PARALLEL RESTRICTED which isn't right
for GUC-related functions either.  Mark isn_weak(boolean) as
VOLATILE and PARALLEL UNSAFE, matching set_config().  Mark isn_weak()
as STABLE and PARALLEL SAFE, matching current_setting().

Reported-by: Viktor Holmberg 
Diagnosed-by: Daniel Gustafsson 
Author: Viktor Holmberg 
Reviewed-by: Tom Lane 
Discussion: https://postgr.es/m/790bc1f9-74dc-4b50-94d2-8147315b1556@Spark