Add an index on pg_inherits.inhparent, and use it to avoid seqscans in

Add an index on pg_inherits.inhparent, and use it to avoid seqscans in
find_inheritance_children().  This is a complete no-op in databases without
any inheritance.  In databases where there are just a few entries in
pg_inherits, it could conceivably be a small loss.  However, in databases with
many inheritance parents, it can be a big win.