When building a list of relations for a parallel processing of a schema
or a database (or just a single-entry list for the non-parallel case
with the database name), the list is allocated and built on-the-fly for
each database processed, leaking after one database-level reindex is
done.  This accumulates leaks when processing all databases, and could
become a visible issue with thousands of relations.

This is fixed by introducing a new routine in simple_list.c to free all
the elements in a simple list made of strings or OIDs.  The header of
the list may be using a variable declaration or an allocated pointer,
so we don't have a routine to free this part to keep the interface
simple.

Per report from coverity for an issue introduced by 5ab892c3, and
valgrind complains about the leak as well.  The idea to introduce a new
routine in simple_list.c is from Tom Lane.

Author: Michael Paquier
Reviewed-by: Tom Lane