pgsql: Add an index on pg_inherits.inhparent, and use it to avoid

Log Message:
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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.

Modified Files:
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pgsql/src/backend/catalog:
pg_inherits.c (r1.3 -> r1.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_inherits.c?r1=1.3&r2=1.4)
pgsql/src/backend/commands:
tablecmds.c (r1.311 -> r1.312)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.311&r2=1.312)
pgsql/src/include/catalog:
catversion.h (r1.562 -> r1.563)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.562&r2=1.563)
indexing.h (r1.112 -> r1.113)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/indexing.h?r1=1.112&r2=1.113)